To be honest I had forgotten it was truth week but I was introducing the idea of Huygen's construction to my class. It's a way of predicting the way a wavefront propagates by assuming that the wavefront is made up of an infinite number of small sources of wavelets. The new wavefront at any position can be found by summing all the wavelets. So I asked them if these wavelet sources really exist. The thing is, if they did then the wavefront would propagate backwards too and it doesn't. Then I remembered truth week. Do they exist? True or false. False. Does it matter? No, provided we get the right answer.