The inside of a cricket ball is made of cork while the outside is made of red leather.
The leather is then stitched together around the centre and this area being called the seam.
This seam is slightly raised and it's this area which fast bowlers use to hit the pitch when it bounces.
With a little help from the pitch, the angle of the delivery will (hopefully) change direction and cause lots of problems for the batsman.
Ever since coloured clothing was introduced in one-day matches, a white ball is used instead of a red one.
