Sleep like a rock
Light as a feather
Cream of the crop
As big as a bus
The above phrases are examples of figurative language, all of which are commonly used in day-to-day English.
Any student--especially any English language learner--can struggle with such figurative speech, particularly when the implied meaning (i.e., idiom) does not translate to the student's first language.
The concept of figurative language is also difficult for struggling readers to understand, but all students need to be able to identify and use it in reading and conversation.