She could tell him she's not well enough to, but she feels strangely compelled to. If she doesn't, he might go on about this really upsetting version. It's a matter of principle. She takes a few moments, but eventually, she starts to sing, weakly, but quite on-key Mary had a little lamb, little lamb, little lamb! Mary had a little lamb, who played by a magic moat. She'd have rhymed it with snow, but he said "goat", so that is what she thinks the original is.

Thanks, Christopher.