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This only works on old fashioned tv screens or computer monitors, so to understand why this happens we need to know how they work. Inside a television there is a big glass chamber which has had all the air sucked out to make a vacuum. At the back of this chamber is an electrical gun which fires electrons towards the back of the screen. The screen is covered with tiny lumps of phosphor, which glows when an electron hits it. If you cover the whole screen with one colour of phosphor, you get a black and white tv.

To make a colour tv screen, they put tiny spots of three different colours of phosphor on the screen in groups. Each group contains a spot of red, a spot of green and one of blue. Lighting these up in different combinations can make all the colours you see on your tv.

As electrons fly towards the screen, they can be moved using a magnetic field - this lets you aim the electrons at the right spot of phosphor and get the right colours in the right place.

When you put a magnet near the tv, it diverts the electrons away from where they should go, and so the wrong phosphor spots light up and you don't get the right colours.

Sometimes, if you put a magnet near a tv for too long, you can make bits of it magnetic and so it will always distort the colours: this is how the colours stay there.

Some tv's have a degaussing coil inside them that re-sets the magnetism when you switch them on, so the colours go back to being correct.

Even the Earth's magnetic field is enough to distort the colours, so if you turn a tv upside down when it's switched on, this can also make the colours go wrong.

We did thisexperiment in kitchen science.