Use round objects like cups to stamp circle prints and notice that a circle is round with no corners.
A circle is a round shape with no corners at all. Cups, lids, and cans are round, so they make great circle stamps.
Dip the rim of a cup in paint, then press it onto paper. Lift straight up to see your circle print.
Mastery looks like: They can stamp a clean circle print and explain that a circle has no corners.
Common stumbles: Sliding the cup instead of pressing straight down and up, which smears the print.