A six-year-old boy died after he fell from an open second floor window as he read a Mr Men book during last year's heatwave, an inquest heard.
William Coy died in hospital following the fall at his Lincoln home in July.
He was sitting on the window sill in his bedroom, which had become "unbearably hot", and was reading his book when he fell to the ground.
Coroner Richard Marshall recorded a verdict of accidental death at the inquest in Boston.
William was found lying unconscious on the concrete patio at the back of the rented terraced house by his sister Lydia, 11, in the evening of 17 July, the inquest heard.
He suffered a severe traumatic brain injury as a result and died two days later.
In a statement read to the inquest, William's father Richard Coy, 37, said his son had gone to bed at about 19:30 BST but 30 minutes later, his daughter asked him why her brother was "asleep outside".
"I opened the back door to see William was laid on the floor. His glasses were on the floor beside him," he said.
"I started to scream and panic and tried to wake William up but he didn't open his eyes."
The Mr Men book was later found underneath a bench on the patio.
The court heard the new UPVC window had been fitted without safety catches.
Lincolnshire Police said it believed William's parents were "very loving towards their children and it was a good family unit", and his death was not suspicious.
Mr Coy, an adult care adviser at Lincolnshire County Council, said his son would "often sit on the window sill of his bedroom and read his books or play with his things".
Both of William's parents were not at the inquest but they described him as their "little hero" and said his organs were donated to help save other people.
Mr Marshall said: "This is one of the most tragic cases I think I have ever dealt with and I add my condolences to the family on their loss."