 Now you see it, now you don't. Hello and welcome to another episode of Do Try This At Home, brought to you by the Institute of Physics. We're making these videos for parents and carers across the UK now and then so that you can explore size together at home with your kids. My name's Taj and I've got another optical illusion for you. This one's called Vanishing Coin. I'm going to hand you over to my niece Saf to explain what we need to do. So what you're going to need is 12 drinks glass with straight edges, some water, a jug, a tea towel, a small jar or dish with a dip to put your coin on. Check if there's enough space for the coin by putting the glass on top. Then for the top of the glass, make a lid or have a look around the house for something that will fit. Fill up your glass with water. This is how much you're going to need. Pour this into the jug. Now all to dry everything off. And you're all set up. All you need to do is call in your family member. Nora, what are calling I can borrow? Don't worry, it won't go anywhere. Now you see it, now you don't. Don't worry, Nora, we'll find it for you. So what's going on here and how you explain it to your family? But let's take the lid off this one and see what Saf wasn't showing Nora. A reflection of the coin. And that's a clue. But there's quite a few bits to this one, so let's break it down. At the start, Nora can see the coin because light bounces off it in all directions and some of it travels in a straight line to our eye. Saf can see it because some of it travels in a straight line to her eye. When Saf puts the glass down, some of the light reflects, but most of it still makes it through. For this trick to work, Saf needs to make all the light reflect. And that's what adding the water does. It completes a five layer structure in which the conditions are just right for all the light to reflect. Light traveling upwards gets trapped, can no longer reach Nora and so the coin seems to vanish. The light can still go out at the top, but Saf had that covered. Don't worry if you don't totally get the signs. Enjoy the trick. Or head over to our website for more information. Well, that's all for this time. So it's goodbye from then and it's goodbye from me.