This web page allows you to visually investigate factors, square numbers and prime numbers. You can use the controls to adjust the number of balls (or squares, if you're using an older browser) and the width of the box. This page looks best in Safari, Opera or Google Chrome as those browsers support the new HTML5 slider control.
If you can make a square, then you have found a square number and the balls turn green. If the balls turn blue, then you have found a rectangle number and two of its factors. If you can't make a rectangle or a square, then you have found a prime number.