A metaphor is a literary technique that compares two things that are not alike. For example, "My love is a rose blooming in my heart", is a metaphor. The literary technique compares love to a rose, two different things that aren't similar.
As I think about an elm tree, I can picture it visually. It is tall; the elm tree provides shade from the hot sun; it is full of leaves; and its roots jut out of the ground at its base. So, turn these descriptions into metaphors:
The elm tree is a tall _______________. (Think of something that is tall.)
The elm tree is a(n)______________________providing shade from the hot sun. (Think of something that provides shade.)
The leaves on the elm tree are a(n)_____________________swaying in the breeze. (Think of something that sways.)
The elm tree's roots are a(n)______________________ jutting/reaching/stretching out of the base of the tree. (Think of something that juts, reaches, or stretches like roots.)
Using metaphors makes your writing more visual and paints a picture for your reader. Using various literary devices like metaphors, similies, and personification will add exciting, creative detail to your writing.