Empty Block

A Used Block appears when Mario or Luigi hits a ? block or a Brick Block with an item inside. Used Blocks are usually square and brown with rounded edges, and in Super Mario World, Used Blocks had small, closed, angry eyes. Also, in Super Mario World if the player hits a P-Switch, all Used Blocks will turn into coins and vice versa for a short time.

Trivia
In Mario & Luigi: Bowser's Inside Story, Broque yells at Mario and Luigi for leaving too many Used Blocks.