What does the saying is a piano you play by mean?

The saying "It's a piano you play by" is a metaphor meaning something is easily understood or done.

Here's why:

* Piano: A piano is a musical instrument that's relatively straightforward to learn the basics of.

* Play by: This implies following a set of instructions or rules.

So, the saying suggests that the thing being described is not complex or difficult to understand or follow, just like learning to play simple melodies on a piano.

Example:

* "Explaining quantum physics to my grandma? That's a piano you play by."

* This means the speaker believes quantum physics is easy to explain, even to someone who doesn't know much about science.

The saying is a playful way to imply that something is simple or obvious.