If your macaron tops cave in when lightly tapped, it means they are hollow and the top has separated from the inside. This could be the result of two things: under baking or over beaten meringue. First, make sure to bake your macarons until they are no longer wet and have fully cooked on the inside. If you’re not sure what to look for, cut one of the macrons open and see if it is still wet inside. Additionally, when beating your egg whites, make sure to beat them just to a stiff peaks (for additional information on over mixed egg whites, see above).