You are taking a leisurely morning swim in a perfectly round lake. Suddenly, when you are right in the centre of the lake – 40 metres from the shore in every direction – you spot a goblin on the shore.
The goblin is very hungry and wants to eat you for breakfast, but it can’t swim. It will run round the lake to try to intercept you when you reach the shore, but it will never enter the water.
The goblin runs 4 times as fast as you can swim. This still isn’t very fast though – you can run much faster.
Can you escape?