Can you find the circumference of one of the round plates in your kitchen cabinet?
This is not easy to do with a ruler, but using a string is a good idea! Turn the plate upside down. Put the string up to the edge of the plate and drag it around until it meets itself again. Cut the string where it meets itself and stretch out the piece of string that was left around the plate. When you measure the length of that piece of string, you have found the circumference of the plate!