Putting like objects together is called matching. Identifying dissimilar objects is called discriminating. The interesting thing about this is that the process of matching requires that children also discriminate. Imagine that you ask the children to put all of the small stuffed animals into a basket during clean-up time. The children who are able to find the similar (small) stuffed animals have also discriminated out the dissimilar (large) stuffed animals.
Finding similarities and differences can happen throughout the day both during free play and during more structured activities. Make sure you have a couple of Memory Games around that vary in skill level as well as many sorting materials that encourage children to look for like and unlike attributes.
Even toddlers can begin identifying like objects if you keep it clear and simple. Provide really easy clues and help guide them by describing the attributes as they go.
How many of you remember this?