If it quacks like a duck, it is a duck.

What rhetorical device is this?

We learned way back in high school many, many years ago the mnemonic:

concave = take a flat surface; curve it inward to make a little cave

convex = take a flat surface; curve it outward to make a sexy breast (Edit: vex rhymes with sex)

My best friend in high school and I would look at our sorely-developing little chests and bemoan the fact that we were concave and not convex.

The meaning has stuck with me for almost four decades, so something in the mnemonic must have worked!