As ever, Geoffrey has hit the nail on the head. As is now, and ever shall be, the first session in the morning will be crucial. England could kick on and push towards 400, or the tail could fold for little more than 300.
Join us in the morning to find out which one it will be.
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Jonny could pulvarize them tomorrow or he could get out in no time and we go for 320-330. Are we going to get Sri Lanka's batting in three innings or the second one at Durham? Sri Lanka need to get Broad's and Anderson's first spell out of the way. We will know more tomorrow night and that will tell us who is sitting pretty."
Bairstow 107* - 164 balls, 13 fours
Earlier we talked about how many runs Bairstow scored square of the wicket, and how few he scored won the ground. This handy little wagon wheel shows as much.
OptaCopyright: Opta
Post update
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Herath can bowl arm balls and straight balls. He is crafty. The day was more about the Sri Lanka seamers. If they had caught Bairstow - and it was a straightforward catch - it would have been different. But he made that chance count. For him it was a steady innings."
Post update
Sri Lanka spinner Rangana Herath on Test Match Special: "Apart from the one chance we dropped, the guys bowled really well and we had a good day altogether.
"It's a good Lord's wicket. There is something for the bowlers but it's still a batting wicket. Bairstow continues his super form. There is nothing so far for the spinners but I don't know how it behave on the third and fourth day."
Was it Sri Lanka's day or England's? "I would say both."
Get Involved
#bbccricket
Scott Clark: England innings bit like my golf: not very pretty, expended more effort than required, ended up not quite where I wanted.
Fraser: Perhaps England's new batting line-up should start Cook, Bairstow, Root, Stokes, Moeen.
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Bairstow sees it as scratchy - we see it as nicely batted. He does score very quick - it's his natural way of playing. And it's a good job he was dropped because England would have been in real trouble. This middle order has got them out of jail."
Post update
Jonny Bairstow on Test Match Special: "It [a century at Lord's] should have happened a few years ago when I got 95 against South Africa. But I am delighted. It was a bit scratchy but sometimes you just have to find a way to get there. There are different pitches and grounds. I have not played at Lord's for a while so I had to get used to it. The ovation at the end was something special."
On the reviewed lbw shout he survived: "That is the DRS system. It is nothing to do with me. The umpire made the decision so I go with that.
"It is an exciting day going ahead tomorrow. But we know how good a start will be. If we can bat well, hopefully into the afternoon, who knows what a good score will be."
Post update
JP, you're right, Bairstow has dug England out of the schtuck today, but he had a little fortune while doing it. There was a drop, a bizarre run-out attempt and a very close lbw shout.
Get Involved
#bbccricket
JP Brett: Jonny Bairstow. More rescues than Jack Bauer.
Post update
More from Jonny Bairstow: "I used to play the ball slightly further out in front of me, but now I play a bit later and underneath my eyes.
"I've always been honest with myself. It's been a journey. I've gone away and worked on things. When w lost 5-0 in Australia, that was a horrible place to be in. I re-evaluated things.
"I've gone about it now how I want to go about it, not how other people want me to go about it.
"I struggled with the slop earlier on, so moved my guard to counter-act that, so as not to fall over."
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Just because Hales gets bogged down he can't slog. It's not a shot. He's got to learn - you can't give it away. If that's his only output, you can't carry on like that.
"Compton - we all know he can play, but to what level? He didn't get his foot over to the line of the ball."
"Someone as determined as Cook will be pretty angry with himself that he got out."
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Sri Lanka bowled some good balls early on but also some scoring balls. After lunch they bowled much better. The line and length was better and they pressured England. That is what got Cook out in the end."
Post update
Jonny Bairstow, speaking to Sky Sports: "It's pretty amazing. After coming five short against South Africa in 2012, it's been a tough four years, but I'm on the board.
"It feels like it should have happened a few years ago. It's a really special moment and I was pretty emotional.
"On 95, I was thinking 'please just bowl me a half-volley'. It is what is it is, now we move on."
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"It was an interesting day and I'm not sure either side has finished the day thinking they're on top. It's a Bairstow day, and then Cook. Cook played really well and showed good judgement and looked nailed on for a hundred."
Post update
Phil Tufnell
Ex-England spinner on BBC Test Match Special
"England will be disappointed. That was a great opportunity for the likes of Vince and Compton to put a marker down and show your Test class - beautiful sunny day, a good wicket. They'll be sitting there thinking, 'I need to have a look at how I'm going about my business'."
Get Involved
#bbccricket
Gareth Price: @jbairstow21 has scored 534 test runs in 2016 at an average of 106. #chapeau.
Post update
That Jonny Bairstow century came right at the end of the first day. You know by now that you can watch all the highlights as they happen right here, don't you? No? Well watch this and see for yourself...
Live Reporting
Stephan Shemilt and Phil Dawkes
All times stated are UK
Get involved
OptaCopyright: Opta 
Latest PostPost update
As ever, Geoffrey has hit the nail on the head. As is now, and ever shall be, the first session in the morning will be crucial. England could kick on and push towards 400, or the tail could fold for little more than 300.
Join us in the morning to find out which one it will be.
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Jonny could pulvarize them tomorrow or he could get out in no time and we go for 320-330. Are we going to get Sri Lanka's batting in three innings or the second one at Durham? Sri Lanka need to get Broad's and Anderson's first spell out of the way. We will know more tomorrow night and that will tell us who is sitting pretty."
Bairstow 107* - 164 balls, 13 fours
Earlier we talked about how many runs Bairstow scored square of the wicket, and how few he scored won the ground. This handy little wagon wheel shows as much.
Post update
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Herath can bowl arm balls and straight balls. He is crafty. The day was more about the Sri Lanka seamers. If they had caught Bairstow - and it was a straightforward catch - it would have been different. But he made that chance count. For him it was a steady innings."
Post update
Sri Lanka spinner Rangana Herath on Test Match Special: "Apart from the one chance we dropped, the guys bowled really well and we had a good day altogether.
"It's a good Lord's wicket. There is something for the bowlers but it's still a batting wicket. Bairstow continues his super form. There is nothing so far for the spinners but I don't know how it behave on the third and fourth day."
Was it Sri Lanka's day or England's? "I would say both."
Get Involved
#bbccricket
Scott Clark: England innings bit like my golf: not very pretty, expended more effort than required, ended up not quite where I wanted.
Fraser: Perhaps England's new batting line-up should start Cook, Bairstow, Root, Stokes, Moeen.
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Bairstow sees it as scratchy - we see it as nicely batted. He does score very quick - it's his natural way of playing. And it's a good job he was dropped because England would have been in real trouble. This middle order has got them out of jail."
Post update
Jonny Bairstow on Test Match Special: "It [a century at Lord's] should have happened a few years ago when I got 95 against South Africa. But I am delighted. It was a bit scratchy but sometimes you just have to find a way to get there. There are different pitches and grounds. I have not played at Lord's for a while so I had to get used to it. The ovation at the end was something special."
On the reviewed lbw shout he survived: "That is the DRS system. It is nothing to do with me. The umpire made the decision so I go with that.
"It is an exciting day going ahead tomorrow. But we know how good a start will be. If we can bat well, hopefully into the afternoon, who knows what a good score will be."
Post update
JP, you're right, Bairstow has dug England out of the schtuck today, but he had a little fortune while doing it. There was a drop, a bizarre run-out attempt and a very close lbw shout.
Get Involved
#bbccricket
JP Brett: Jonny Bairstow. More rescues than Jack Bauer.
Post update
More from Jonny Bairstow: "I used to play the ball slightly further out in front of me, but now I play a bit later and underneath my eyes.
"I've always been honest with myself. It's been a journey. I've gone away and worked on things. When w lost 5-0 in Australia, that was a horrible place to be in. I re-evaluated things.
"I've gone about it now how I want to go about it, not how other people want me to go about it.
"I struggled with the slop earlier on, so moved my guard to counter-act that, so as not to fall over."
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Just because Hales gets bogged down he can't slog. It's not a shot. He's got to learn - you can't give it away. If that's his only output, you can't carry on like that.
"Compton - we all know he can play, but to what level? He didn't get his foot over to the line of the ball."
Post update
Jonathan Agnew
BBC Test Match Special
"Someone as determined as Cook will be pretty angry with himself that he got out."
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"Sri Lanka bowled some good balls early on but also some scoring balls. After lunch they bowled much better. The line and length was better and they pressured England. That is what got Cook out in the end."
Post update
Jonny Bairstow, speaking to Sky Sports: "It's pretty amazing. After coming five short against South Africa in 2012, it's been a tough four years, but I'm on the board.
"It feels like it should have happened a few years ago. It's a really special moment and I was pretty emotional.
"On 95, I was thinking 'please just bowl me a half-volley'. It is what is it is, now we move on."
Post update
Geoffrey Boycott
Ex-England batsman on BBC Test Match Special
"It was an interesting day and I'm not sure either side has finished the day thinking they're on top. It's a Bairstow day, and then Cook. Cook played really well and showed good judgement and looked nailed on for a hundred."
Post update
Phil Tufnell
Ex-England spinner on BBC Test Match Special
"England will be disappointed. That was a great opportunity for the likes of Vince and Compton to put a marker down and show your Test class - beautiful sunny day, a good wicket. They'll be sitting there thinking, 'I need to have a look at how I'm going about my business'."
Get Involved
#bbccricket
Gareth Price: @jbairstow21 has scored 534 test runs in 2016 at an average of 106. #chapeau.
Post update
That Jonny Bairstow century came right at the end of the first day. You know by now that you can watch all the highlights as they happen right here, don't you? No? Well watch this and see for yourself...