It’s raining cats and dogs!

The expression (they say) comes from England in the Middle Ages where the peasants made the roofs of their houses out of straw. Dogs, cats and mice would sleep on the roof which was the warmest place they could find (they weren't allowed in the houses). When it rained a lot, the mice would fall though the holes in the straw ceiling. When it rained heavily, the cats and dogs fell through. If a peasant were to look out it would have seemed it had been raining cats and dogs!