I like the
advisory page for their new gallium coins. Pure gallium's melting point is 30°C (86°F) so a gallium coin will melt on a warm day, under bright lights, or in your hand.
I wonder if they'll ever try to strike mercury coins. Mercury (also known as quicksilver) melts at -39°C (-38°F), and a coin made of mercury would still melt in your freezer. You'd need to store them in dry ice or liquid nitrogen to keep them solid.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis