Super Mario Bros. has 10 “1UP” blocks throughout its eight worlds. One of them is in World 1-2, and another is in world 8-2. These are visible (although they appear to be ordinary bricks when hit), and always produce a 1UP Mushroom when hit.
But also, there are a total of eight other 1UP Mushroom blocks in the game. These are hidden, invisible and intangible unless struck from beneath. That causes them to appear and produce a 1UP Mushroom… sometimes.
The 1UP blocks are always in the first level of each world (1-1, 2-1, 3-1, and so on.). They also always appear if you start on that level (say, using the level selection feature unlocked after finishing 8-4) or if the player has just warped to that level. But in other situations, the block usually doesn’t appear.
What causes them to appear? Even Nintendo’s guide authors at the time didn’t know. Depending on the source, it either had to do with whether Mario was big when he hit it, or whether he found all the coins in the previous world.
Stephen Lindholm’s website has the details on the true criteria, and you should read it there, but to summarize:
- There is a flag in the game that is set when a game is begun, when the player warps, or if the player has collected a certain number of coins in the third level of the previous world. 1-3, 2-3, 3-3, etc. That is the deciding factor.
- How many coins is needed? In World 1-3, it’s all but two. In all the other X-3 levels, you must collect every coin! The game has a short table in its code for the minimum number of coins necessary to earn the mushroom on each level.
- If you do this, the flag is set. When you get to the following world’s first level, when the 1UP Mushroom block would be generated by the game’s world building code (which operates off the screen to the right of the player’s location), if the flag is set, the invisible block is placed and the flag is unset.
- If the player dies before collecting it, or never hits it, either way the mushroom is lost.