The name commonly used to describe the
enigmatic,
floating blocks in almost all of the
Super Mario Brothers games.
Any high-impulse collision between the bottom of a question mark box and another object will result in the question mark box releasing either money or a useful object, such as a mushroom which will cause you to grow to twice your size, or a flower which will allow you to summon and throw fireballs.
Sometimes a question mark box masks its true identity, and appears to the world as just another perfectly ordinary floating block of bricks. Because you can never be certain which blocks may secretly be question marks in disguise, it is important to smash any normal blocks found, just to be certain. This is made especially important by the fact that disguised question mark boxes, unless they contain money, often contain the most valuable items you could possibly find at that moment, such as a star or a magic beanstalk.
Moral: if you come across any bricks, be sure to smash them against your head.