What is Thunder?
Thunder is the sound that lightning makes. Any sound you hear is made up of vibrations and the vibrations travel through the air as waves until they reach your ear.
This means lightning must cause some vibrations.
Lightning is a huge discharge of electricity. When lightning strikes huge amounts of electricity shoot through the air, which causes two things to happen.
1. The electricity hits the air and starts it vibrating, anything vibrating causes a sound.
2.The lightning is also very hot and heats up the air around it. Hot air gets bigger and expands. Because the lightning is so ridiculously hot the air gets bigger very quickly and pushes against the air particles starting another vibration.
These vibrations are what you are hearing when you hear thunder.