Start with a trip to a pet shop or animal shelter where you can see lots of dogs, cats, fish, or rabbits up close. Or, try a trip to the aquarium.
Discuss the different animals. How are all the dogs, cats, fish, or rabbits the same? How are they different from each other? Think about physical characteristics like size, shape, color, and fur texture, as well as other qualities like smell, and temperament.
As an extension activity, observe and talk about other species of animals; some will have little individual variation among them (squirrels and pigeons for example), others will have more.