From the air track to the particle collider, the Principle of Conservation of Linear Momentum (POCOLM) is one of the most useful tools we have to predict, explain or analyze the outcomes of an exchange of force. I felt compelled to make this particular collision more interesting than other intro-to-POCOLM classics: bodies-on-ice (ooooh) or particles-in-space (aaaah). So here we have two fish colliding in an inviscid lake (one of the lesser prominent locales of Physicsland). Enjoy!