Emily was delighted when she found that frogs had settled in the garden pond. Sometimes there was a big green one, sometimes a brown, and she saw a couple of little ones too.

Then she got to wondering just how many frogs actually lived in the pond. She tried to count them but could never reach the same total twice. Once, she was quite sure there were six when, without warning, a completely new frog appeared.

Without draining the pond what would be the best way of finding how many frogs there are?

Henrik Ibsen, 1828 – 1906, Norwegian dramatist, wrote: ‘What’s a man’s first duty? The answer’s brief: to be himself.’