The eggshell is made of calcium carbonate.When you submerge the egg in vinegar, the acid in the vinegar reacts with the calcium carbonate and produces carbon dioxide.This chemical reaction is what causes the little bubbles you see surrounding the egg. As the shell dissolves, the thin membrane of the egg is left behind.The membrane is semi-permeable, which means the vinegar can pass through it.The egg absorbs the vinegar, and that’s why you see the egg increase in size as it continues to sit in the vinegar.