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 and the width of the box to factorise numbers and find out whether they are squares or primes.
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.