High-speed footage of rubidium reacting explosively as it falls into cold water. Rubidium (Rb) is a highly-reactive alkali metal element that reacts with water to produce rubidium hydroxide and hydrogen gas. In this demonstration the force of the initial reaction throws the remaining rubidium into the air in a number of pieces, which ignite on contact with oxygen in the air and fall as burning blobs of molten metal. Filmed at 2000 frames per second (slowed down 80 times).