If you don't know the value of PI, but you do know:

• The area of a circle is PI * r * r, if r is the circle's radius
• The area of a square is 2r * 2r, if r is 1/2 of the length of the square's side
• How to throw darts so they land randomly in a square (they must land somewhere in the square, so you have to be a good dart thrower)

How?

• Build a square that is 2 feet on each side.
• Centered in this square, place a dart board (a circle) that is 2 feet in diamater.
• The square has area 2 * 2 = 4 square feet
• The circle has area PI square feet
• The probability of a randomly thrown dart landing in the circle is the ratio of the circle's area to the square's area.
• Throw a bunch of darts randomly into the square.
• Count the fraction of them that land in the circle.
• That number approaches PI / 4 as you throw lots and lots of darts.

This is a good example to

• Show how computers can simulate real life
• Show that computers can, well..., compute things