This is the Fish Hammer built by mad scientist Neil Mendoza (very detailed Instructable to build your own HERE). It uses a camera to track a goldfish's movement inside a tank, which moves the hammer on the tank's exterior to the fish's position along a semicircular track. I'm not sure what actually activates the hammer's smash motion though. Maybe the fish staying still briefly? I don't know. And, if we're being completely honest, I don't care. It's Friday afternoon and all I can think about is-- "Happy hour?" What? No. You -- all I can think about is you. I'm going to miss you this weekend. "Wish I could say the same." You really mean it?! Rub this lamp and tell him the same thing you told me.
Keep going for the video.
Thanks to n0nentity, who agrees you should never trust a fish do a carpenter's job.