How do you quickly find which card is missing from a deck? Arrange it by suits and number. (I actually do this with several decks and give extra credit based on the time it takes to find the missing cards. The students love it.) Cards are grouped by two characteristics at once. I call this "Cross Classification," much like the cross referencing in a library catalog. Specifying suit and number zeroes in on a specific card. The Periodic Table is a similar cross classification scheme. Specifying number of shells and number of valence electrons (including "d" electrons for transition elements) zeroes in on a specific element. This Powerpoint presentation introduces the concept and value of classification using several examples, culminating in the Periodic Table.
Free worksheets are available on this topic on my website at http://evanschemistrycorner.com