To see how you can make lots and lots of different molecules of RNA just by rearranging the four purines that RNA is made out of, try building a tower out of four different colors of Legos. You could just alternate the four colors – blue, yellow, green, red, blue, yellow, green, red – but you can also make different patterns. How many different patterns can you (or your class) come up with? It’s okay to have two or more Legos of the same color together.
This tower is only one Lego thick. RNA also is only one Lego thick. How high can you build a tower only one Lego thick? Does it tend to break and fall down? This is a problem for RNA too – it can’t make very long strings, or it breaks.
(If you don’t have any Legos, you can draw pictures of Lego towers using crayons in four different colors).