These cards can be sorted by subject (dog, cat, flower) or by color (blue, green, red/pink). They are based on cards mentioned in Mind in the Making (p. 53, 197): the Dimensional Change Card Sort Task developed by Phillip David Zelazo at the U of Minnesota. This activity is designed to stretch the working memory, as well as use cognitive flexibility and reflection. All of these help build focus and self-control. I took all of the pictures myself. One of my friends said that these are the favorite cards in her toddler room.
Examples of use: Ask two-year-olds to put all of the flowers (or dogs or cats) in a pile. Or to sort by a color.
Ask three-year-olds to put the flowers in one pile, the dogs in another, and the cats in a third. Or to sort the cards into piles by color. Or see what they do with no instruction.
Ask four-year-olds to first sort by subject and then by color. Three-year-olds cannot usually switch the rules like this, but most fours can.