When writing a small school project on Pierre Curie, I came across his following theorem. I thought it was so cool at the time, that it immediately stuck in my mind. It took many years until I understood a word of it. :)

The paramagnetic susceptibility is inversely proportional to the absolute temperature.

Saying it now, after my engineering degree, it doesn't sound quite as impressive as it did when I was 14, but still... :)