Magic Ball

A Magic Ball, also known as a Crystal, is an item in Super Mario Bros. 3 and Super Mario World. It looks like a round power-up object with a question mark on it, and is required to clear certain levels. It automatically activates when Mario or Luigi touch it.

Super Mario Bros. 3
Magic Balls first appear in Super Mario Bros. 3. They only appear in fortresses after the player defeats Boom Boom. Upon his defeat, Boom Boom will drop a Magic Ball, and Mario must touch it to destroy the fortress and therefore complete the level. If Invincible Mario defeats Boom Boom by touching him, the Magic Ball will appear upside down, in the center of the arena. They later appeared in the game's remakes, playing the same role.

Super Mario World
Only one Magic Ball appears in Super Mario World, and that is in the Sunken Ghost Ship (this is most likely to give this level the feel of Super Mario Bros. 3, as it is suggested to be one of the Airships from that game). Touching it beats the level in a similar fashion of beating a boss, but it is left out in the open instead of being held by a mini-boss.

Super Mario-Kun
In volume 4 of Super Mario-Kun, whose arc is based on Super Mario World, Mario grabs the Magic Ball to exit the level, and he and his friends are later found on the top of the entrance to the Valley of Bowser.