Shove your fingers into a bowl of shrimp to select the best one. You know your shrimp and you know them well, so you won't settle for anything less than the best. Put your whole arm in there and feel around until you can select the one you want most.
You do this at every restaurant, party, and indoor sporting event and people start getting wise to your shrimp selection methods. One day, as you attempt to select your shrimp, a sharp pain shoots through your finger. Ow! You pull your hand back to find... a tack (たく)!