In everyday life, there are three states of matter - solids, liquids and gases. The differences between the three states are due to the arrangement and spacing of the particles and their motion.
The particles in a solid:
The particles in a liquid:
The particles in a gas:
Adding or removing energy from a material can change its state. Heating a solid material will cause it to melt from a solid to a liquid. Continued heating will cause the liquid to boil or evaporate to form a gas. In some instances, a solid material can go straight to being a gas without first becoming a liquid when heated. This process is called sublimation.
Boiling is an active process. People actively apply energy to a liquid to turn it into a gas using a heater such as a kettle.
Evaporation on the other hand is a passive process. The liquid slowly absorbs energy from the surrounding area so that some of its particles gain enough energy to escape the liquid.
Throughout all of these changes the number of particles does not change, just their spacing and arrangement. As a result the total mass does not change.
These changes in state are called physical changes because the process can be reversed (eg by cooling instead of heating). This is different to the changes seen in a chemical reaction, which cannot be reversed so easily.