A metaphor is also a form of comparison, but it does not use "like" or "as." For example: That last math problem is a bear!
Similes and metaphors seem to often be taught in the spring, right before state tests, judging from the number of hits we get on our Poetry Friday post, "Poetry Friday -- Simile and Metaphor Poems." Why not start exploring these forms of comparisons with your students NOW, so they can be using them in their writing all year long and have the difference between the two of them internalized by April??
Here are two great books that explore metaphors and invite you to write more. Both are personal copies.

by Bernard Waber
Houghton Mifflin Books, 2002
"Courage is being the first to make up after an argument."
"Courage is being the first to make up after an argument."
"Courage is breaking bad habits."
"Courage is a blade of grass breaking through the icy snow.
"Courage is a blade of grass breaking through the icy snow.
by Harriet Ziefert
illustrated by Jennifer Rapp
Blue Apple Books, 2005
"Misery is when your mom insists on sensible school shoes and you really want flip-flops."
"Misery is two against one -- especially if you're not part of the twosome."
"Misery is learning that you and everyone in your class will be checked for head lice!"