Compares two things using "like" or "as"

The milk was as curdled AS cottage cheese.

Creates imagery of lumpy, stinky milk.

Says that one thing IS something else

"There's a fire in my heart and you fan it, Janet." (Rocky Horror Picture Show)

Emphasises the passion Rocky feels for Janet. (His heart aches like it is on fire. Her beauty makes it hurt/burn more.)

When the first sound of two or more words is the same.

Peter Piper picked a peck of pickled peppers.

The repetitive plosive /p/ sound makes Peter sound quite busy Picking those Peppers! (like fast breathing/exhaling.)