You will need:

What to do:

Before setting up this demonstration it’s important to know that it only works if you’re looking at it from the right place. Either set up a chair for your family member, or if you’re doing it online position the camera on your laptop or phone, so you know where the demo will be viewed from. Now you’re ready to get started.

Quick version:

1. Draw two short horizontal arrows, both pointing the same way, on your piece of paper, they should be about as long as your glass is wide (like in the image below)

2. Find a way to stand up your paper – fold the sides, lean it against a wall, box or book.

3. Put the glass in front of your arrows, and pour water into your glass until the level of the water is above one arrow but below the other.

4. Move the glass towards and away from your chair/webcam until you find the spot where the bottom arrow is reversed and the same size as the top one.

5. Once you’ve worked out where that is, mark the position, pour the water back into the jug and put the empty glass back into position. You're now set up.

6. Get you family in position, in person or online.

7. To perform the trick, all you need to do is pour water into the glass and watch their amazed faces as the arrow reverses before their eyes.

Genius challenge:

1. Can you draw a ray diagram to explain why this happens?

Show your science teacher to get genius points!