Animals with wings (that fly):
bats
butterflies
hawks
mosquitoes

Animals with four legs (that walk on the ground):
cattle
dogs
elephants
mice

Animals with streamlined shapes (that live in the water):
cod
dolphins
goldfish
trout

I hope your list is similar to mine. The animals in all three groups share similar body characteristics that are analogous structures (wings, legs, body shape) because they share a similar lifestyle or a functional need (which I listed in parentheses in the lists above).
Classifications like these are easy to make but are not very informative. Perhaps you felt it was too superficial and didn't take into account underlying similarities.

Modern biologists prefer to classify organisms according to their ancestry using homologous similarities. Homologous similarities imply a shared ancestry and these similarities cannot always be explained by functional needs or similar lifestyles. Indeed, most homologous similarities can only be explained by ancestry. Homologous characteristics may be useful to the organisms' lifestyle or not useful at all. They might even be a wee bit of a hindrance! But homologous characteristics are useful to biologists because they direct our attention to the ancestry and therefore the evolution of the organisms.
Homologous characteristics (similarities) are like a family heirloom handed down through the generations. Sometimes they are just old "junk" and sometimes they are precious "antiques". Regardless, biologists use homologous similarities to classify organisms and to highlight the ancestry among the groups.

Here's that list again. Now try to arrange these animals according to what you know (or may not know) about their ancestry.
bats
butterflies
cattle
cod
dogs
dolphins
elephants
goldfish
hawks
mice
mosquitoes
trout

This isn't as easy to do as before. Indeed, the "trick" to homologous classifications is to overlook the analogous similarities in order to concentrate your attention on the homologous similarities. Go through that list and group the animals according to similarities caused, not by shared lifestyles, but due to shared ancestry.

I've come up with four different groups and some of the groups have more animals in it than the others.
Give it a try and then go to the next page to see my answers.