Why does vinegar make an egg bounce?

Answer One

Here's what happened:

Several things happened to the eggshell while it was soaking in the vinegar. The vinegar is an acid and it caused the shell material to breakdown into two different materials. One is carbon dioxide bubbles that were released into the air. The other material is lime. The lime remains in the solution. Also, the appearance of the shell changes asa result of this chemical reaction. This is a fairly common process.It happens in nature a lot, creating limestone caves. This chemical reaction also attacks your teeth. Yuck! Makes you want to go brush your teeth!

Answer Two

Soaking an egg in vinegar dissolves the calcium carbonate in the shell* leaving the flexible membrane. The egg might bounce but will more likely simply splatter.

*Acetic acid reacts with calcium carbonate to produce carbon dioxide gas and water-soluble calcium acetate.