Some shoes have hard, smooth soles and are treacherously slippery. Others have moulded patterns and rubber that grips to give you good traction. Think of tyres as your car's shoes, and the tread as the sole. The grooves in the tread are specially designed to remove water from under the tyre in wet conditions. As the tread wears down, the tyre's ability to remove water gradually reduces. Consequently, a 'bald' tyre is very dangerous in the wet.

The minimum legal tread depth on a tyre is 1.5mm - about the height of the head on a match. On the side of your tyres you can find little arrows pointing to tread wear indicator bars. When the tread wears down to this level, the tyres need replacing.