The Ice Melting Blocks look similar, but are composed of different materials. One block feels cold to the touch while the other block feels slightly warm. Both blocks are at room temperature but have very different thermal conductivities.
After allowing students to hold the blocks, ask them which block would melt ice more quickly. Place an ice cube on each block and watch their amazement as the "cold" block melts the ice cube within 2 minutes. The melting of the ice cube is barely noticeable on the "warm" block. The "cold" block is aluminum and has a much greater ability to transfer heat to the ice cube or the hand. The "warm" block is plastic, which does not conduct heat well.