[tcskeptic]

I don't think that is all there is to it. The page says they are filled with "Phase Change Material" (which I would guess is some sort of wax) that melts at !40F. I don't know why that would be better or worse than a lump of metal though.

[jasonkester]

Apologies for going meta, but it's refreshing to see that I'm not the only one who uses capital 1's from time to time.

[revolvingcur]

It's much more likely to be a fusible alloy such as Wood's metal or Field's metal. My guess is that it's closer to Field's metal, since it is non-toxic and has a melting point very close to their advertised ideal.

[gridspy]

Good guess about the wax.