The infinite loop.
The set of all points equidistant from a center point.
The wheel is arguably the most important invention in human history, entirely based on the circle.
Cherry Pie is Delicious (C=πd). Apple Pies are too (A=πr²).
Plain English: A circle is the perfect shape. Pi (3.14) is the magic code that unlocks it. If you know how wide it is, multiply by Pi to see how far it is to walk around it.
Real-world example: Tires. A bigger tire covers more distance per rotation because its Circumference is larger.
Mixing up Radius (halfway) and Diameter (all the way). Formulas usually ask for r, but problems give you d.