Totally Awesome to Watch!!! This is one of the most striking science demonstrations we have seen in a long time. Place an ice cube on each of these two identical looking black blocks at room temperature. One ice cube instantly begins to melt and is totally gone in about 90 seconds while the other ice cube shows no evidence of melting. Great for showing the difference in heat conductivity in different materials. This demo is simple enough for elementary students, yet also challenges high school and university level students. A truly beautiful demonstration! Invented by retired teacher Jim Housley. Set of two blocks. 9.5 x 9.5 cm (3.75" x 3.75") each.