ELI5: How do warm-blooded animals generate heat? How do cold-blooded animals *not*?

Hi folks,

My daughter and I came across a lizard sunning itself today on a walk, and I began to explain cold-blooded and warm-blooded to her. I realized, to my surprise, that I couldn't get very far beyond "cold-blooded animals don't produce their own heat, warm-blooded animals do". I've since done some wikipedia research, as well as searching on this sub, but I can't find anything explaining why the blood and related systems of warm-blooded animals produce heat, and why the blood and related systems of cold-blooded animals do not.

Can anyone help me? You can bump up the explanation if you want, I'm trying to explain it to a 7-year old. ;)

Thanks, y'all!