A university expert has warned people against putting hot water in birdbaths - because it freezes more quickly.

Dr Len Fisher, from the University of Bristol, also advised drivers to use cold water to remove frost from car windscreens.

He carried out an experiment for the BBC's Today programme which proved the theory that hot water freezes faster than cold water.

Over a controlled period of time, hot and cold water were left to freeze in mixing bowls with the stated result.

Dr Fisher: 'The water goes round and round'

Dr Fisher, who is an honorary research fellow at the university, told the Today programme that there were many theories why hot water freezes faster.

He said: "Water in hot containers evaporates, so by the time it gets to freezing there is less water to freeze.

"One hypothesis I favour is that with hot water, you get convection currents like above a radiator.

"The water goes round and round, the hot water rises in the middle, cools and goes down the side, leaving a 'skin' on the top which freezes."