How is the following excerpt from Tuck Everlasting by Natalie Babbitt a simile?"and it was if they had slipped in under a giant colander"

This is a simile because the author makes a comparison between  two unlike things using the words “like” or “as.” In this case, the author is comparing the feeling of being under the treetops to the feeling of being under a giant, overturned colander. 

A colander is a bowl with small holes in it. It’s usually used to drain pasta or lettuce. You probably have one in your kitchen! If you can imagine the terrifying situation of being trapped under a giant colander, you can see why the comparision actually works. The small holes would only let in tiny pinpricks of light, resulting in an effect that some might call “dappled light.” Likewise, the trees over Winnie’s head are so thick and old that they only let in little tiny dots of light.

This simile creates a romantic, mysterious atmosphere.