The Rainbow Effect of Petrol on Water

When petrol is still on the ground and it's mixed with water, how comes it so colourful? You get that wonderful rainbow effect, don't you? Yeah. And have you actually tried doing this yourself, pouring some oil and testing other oils or is it just petrol you've tried? Well, I've just seen petrol. You haven't put any matches near it, have you? Good man. No, the reason for this is that oil, when it goes on water, you know what happens to oil on water? Yeah, the oil floats on top of the water. It's not spot on, the oil float. But when it floats, it doesn't just form globules