Thunder and lightning happen when there is a buildup of electrical energy in the atmosphere during a storm. The storm clouds move around and create friction between each other, which causes the buildup of this electrical energy.

When the electrical energy becomes too strong, it is discharged in the form of lightning. Lightning is a giant spark that travels between the negatively charged bottom of the cloud and the positively charged ground.

The air around the lightning bolt heats up very quickly and expands rapidly, creating a shock wave that we hear as thunder. This is why we see the lightning first and then hear the thunder a few seconds later.