Nintendo
So, what are players doing with the toolset provided by Nintendo? Amazing things, as evidenced by these five levels full of game-created tunes.
Wondering how it all happens? It's the music blocks! These guys:
Nintendo
These levels have become especially popular in the early days of "Super Mario Maker," which launched on September 11 for the Nintendo Wii U. It's easy to understand why: they're simple, interesting, and filled with nostalgia (as if the game wasn't already packed to the gills with nostalgia).
But despite the seeming simplicity of these levels, creating them is amazingly complex. You can't just program a music block to produce a desired note. The blocks only create a note when they're interacted with; the pitch of that note is dictated by where it's placed on-screen. Here's a helpful chart from GameFAQ explaining the system:
Nintendo
As the pirahna plants fall, they interact with the music notes. To create timing for how these notes will play, you need to force consistency in movement on the level. One such solution is putting constantly moving tracks at the bottom of the level (which keeps Mario in movement). Another such solution is forcing Mario to run full speed throughout the whole level in order to survive.
In both cases, the goal is to keep Mario in a constant state of movement, thus providing a consistent time signature. Think of it like a record player that runs at a constant speed, thus providing a consistent rate of music. Except, instead of a record player, it's a little Italian plumber trying not to fall to his death.