Ask a stupid question....

Okay, the is a stupid question. We all remember being told that mammals and birds are warm blooded and that amphibians, fish, and bugs are cold blooded. My question is....why are they cold blooded. I figure that most animals' cellular structure is similar to each other, especially animals with a circulatory and nervous system. Humans produce heat by our chemical reactions in our cells....don't fish do that as well? If not, what about this. When we are cold, we shiver. The rapid contractions of our muscles produce heat to warm us up. Also, any movement produces heat in our bodies (exercise, walking, even getting up out of chair). Since cold blooded animals for the most part have skeletons and muscles similar to ours, wouldn't they produce heat as well? So, my only other thought is that both phylums can create heat, but perhaps we are labeled warm blooded due to our ability to retain heat. Anyway, does anybody actually remember learning not just the "what" but the "why"?