The whole phrase has a job
An idiom is an expression whose meaning as a whole differs from the meanings of its individual words. Knowing each word is not always enough to work out what someone means.
Cambridge Dictionary describes this phrase-level meaning. The words arrive together with an established use of their own.
Where is the cake?
Imagine a friend says, Putting up that tent was a piece of cake. You would not need to look for icing on the campsite.
A piece of cake is an informal way to describe something very easy to do. Our friend found the task easy; the tent has not become edible.
Same words, different setting
Now imagine someone points at a plate and offers you a piece of cake. This time, reaching for a fork makes perfect sense.
The words are the same. The situation helps you decide whether the speaker means an actual slice or something easy. Reading only the word cake would miss that difference.
A little language surprise
When a phrase seems odd, try looking it up as a whole expression. A dictionary entry can reveal a meaning that separate word definitions would not explain.
For another twist in English, meet words that can mean their own opposite. Or explore more ways words surprise us. The next puzzle might contain more than one kind of clue.