I loved Theo Gray's frozen mercury fish but, as he says, mercury is bad for you. If you want to play with a nontoxic metal that melts at low temperatures, you can buy little bottles of it at scitoys.com. Simon Field, the proprietor, sells two kinds.
In the photo above, I am holding two small vials of liquid metal. The vial on the right contains gallium, an element that melts at 29.76° Celsius (85.57° Fahrenheit). The vial on the left is an alloy that contains gallium, indium, and tin, and melts at -20° Celsius (-4° Fahrenheit).You can do a lot of fun things with these. For instance, you can put a drop of gallium on a sheet of aluminum foil and it will combine with the aluminum, dissolving a hole in it. Nontoxic metal alloy that is liquid at room temperature
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects