There's a company called BARB that measures audiences for TV programs.
They select a sample of households that is supposed to be a representative cross-sample of the whole country.
Each of these households is given a box that records what programme is being watched on a minute-by-minute basis and this info is then reported back.
When someone living in the house comes into the room, they have to press a button on a handset that says they are watching the television.
From the info collected, they can estimate how many millions of people - if that - are watching in the country.
