Lightning is a discharge of static electricity from a thunderstorm.
The negative charge at the bottom of the cloud forces negative charges in the ground to migrate away from the storm leaving a net positive charge to the ground below the clouds. When these static charges become too great lightning occurs momentarily neutralizing the charges. This heating and cooling creates a shockwave in the air which we hear as thunder. Observe the students as they put their jars together. Cut 2 pieces of foil a little taller than your cup and long enough to reach around it with about a inch overlap.