Anyone for tennis?

That happy time has come when we put a tennis ball in our precious birdbath to prevent it cracking in the freezing weather.

The last few days have seen the temperature plummet to minus 1 or 2 degrees overnight.

So in goes the tennis ball.

There are a few theories about why this works; I favour the one which assumes that as water freezes, it expands and the ice squeezes the tennis ball rather than cracks the dish.