Question

explain what causes lightning and thunder.

Answer 1

Summary only: Lightning is caused by clouds of one charge coming close to clouds of another charge or by clouds with one charge discharging to earth. The tremendous heat generated by the lightning pushes the air molecules apart; when they come back together, that energy is dissipated in the form of thunder.

Here is a site that explains it in quite a bit of detail.
http://en.wikipedia.org/wiki/Lightning