Various grammatical constructions are used in English to compare things. as … as is used to compare things which have similar or identical qualities. There are many idiomatic phrases in English which use this construction, for example, as stubborn as a mule (very inflexible). These expressions are used to make the language more vivid.
As a non native speaker of English, it is sometimes safer to steer clear of (avoid) using idioms as, when used incorrectly, they can confuse or, even worse, cause a misunderstanding. However, especially when dealing with native speakers, it can be helpful to understand some idiomatic language, or at least to be able to identify that the words you hear are not to be translated literally.
These tasks deal with some of the most common idioms of comparison.