Why do squirrels swim on their backs?

Went to a quiz night with friends ... and surprisingly enough - we won! This is my share of the winnings.

The theme was 'Arts and Entertainment' but it didn't take itself too seriously. There was only one 'Matisse' question and one 'Shakespeare' question. Admittedly, everyone struggled with 'what is the current number one song?', whereas everyone managed the final question in the 'comedy' category:

Why do squirrels swim on their backs?

Answers on a postcard ...