To convince Churchill of Pykrete's qualities, Mountbatten put a block of Pykrete into Churchill's bath (while he was in it, according to the Wikipedia article):

What happened next was explained several years after the war by Lord Mountbatten in a widely-quoted after-dinner speech. "I was sent to Chequers to see the Prime Minister and was told he was in his bath. I said, 'Good, that's exactly where I want him to be.' I nipped up the stairs and called out to him, 'I have a block of a new material which I would like to put in your bath.' After that, he suggested that I should take it to the Quebec Conference." The demonstration in Churchill's steaming bath had been most dramatic. After the outer film of ice on the small pykrete cube had melted, the freshly exposed wood pulp kept the remainder of the block from thawing.

— Pyke, the Unknown Genius, [8] Lampe, David