Teaching students to understand idiomatic expressions is complicated by two factors. First, idioms are often cliched examples of other figurative language techniques. For example, "Reminding Tommy to stay in his seat was like beating a dead horse." This idiom is actually a simile. The only distinction that this simile has is that it has been used so frequently that it is now cliched and, hence, an idiom. The second complicating factor is that most elementary students have had little exposure to the many antiquated idioms floating around in the language. If a student cannot recognize that an expression is commonly used in language art large, that students will have difficulty identifying (and understanding) idioms.
However, I do not believe that these complications are insurmountable. The best way to prepare students to recognize and interpret idioms is to give them exposure to idioms. Given enough leading context, thoughtful students should be able to decode the most cryptic and archaic idioms. Fortunately, I've gathered many of the commonly used idioms and presented them into double-sided worksheets with which students can gain more practice identifying and interpreting idioms. Visit my website now for free idiom worksheets.