Why is it that sometimes when you wee it goes frothy?

The thing that makes that happen is protein. Sometimes, if you have certain problems with the kidneys you can produce very frothy urine: it's almost like champagne. You get this massive head on the urine. That's a serious problem but the day-to-day occasional frothiness is just because occasionally there's more protein in your urine than others. The protein breaks down the surface tension of water and you get these bubbles forming. It acts a bit like bubble bath which is why you get the fizz.