Circles are amazing shapes. Every single point on a circle is the same distance from the centre. Also they are heavily involved with the irrational number which we call pi. Pi has a never-ending decimal tail that never shows a pattern.

The diameter fits around the outside 3 and a bit times (π):

Or, if you want to use the diameter:

There will be a π button on your calculator.

Again, pi is involved in the area formula: