Scientists used an underwater video camera to record the behavior of octopuses and discovered that they throw things at each other (and attempt to dodge the projectiles). Some of the items Octopuses hurl include shells, silt, and algae, which they give an extra boost using a jet of water from their siphon.
From Nature:
The team found clues that the octopuses were deliberately targeting one another. Throws that made contact with another octopus were relatively strong and often occurred when the thrower was displaying a uniform dark or medium body colour. Another clue: sometimes the octopuses on the receiving end ducked. Throws that made octo-contact were also more likely to be accomplished with a specific set of arms, and the projectile was more likely to be silt.
