Easier said than done
Something is 'easier said than done' if it's more difficult than it sounds
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“Easier said than done” is something you can say when a situation seems easy, but it’s not. For example, the key to online marketing is to build a big audience and make them targeted offers. Well, that’s easier said than done: it’s hard to build a big audience these days!
To lose weight, you just have to consume fewer calories than you burn each day. For a lot of people, that’s easier said than done: you don’t know how many calories you burn, exactly, and it’s hard to control calories if you can’t prepare every meal.
In both these cases, the reality is more complicated than it sounds. Or, put another way, the short description makes something sound easier than it is.