Punch cards.
This technique derives from the punched cards of "computers" so we immediately connected to the electrical circuits on-off, but that's another story. We now return to shells. On each tab, we glued a picture of a shell and we, with scissors, "open" to the outside hole corresponding to each characteristic possessed by the shell in question. How to use the punch card? Imagine having to look for a shell of brown color. Collect the cards in one deck and strung a stick (good ones for kebabs) in the hole that corresponds to the characteristic "brown". Hold the deck in the air: all cards with the characteristic that will fall because the corresponding hole is "open", while the other will remain suspended. Scartatele and collect the cards in the deck falls. Now continue with another feature. You will see that eventually there will be only one form: that of refined shell.