Why Ant Won't Get Tired?

A kid asked me last few weeks ago, "Uncle, why ant does not get tired by standing / walking the entire day, but you get tired easily but standing for a while?"

I thought for a while and reply him, "That's because ant is a bug and we are human! Ant is designed to stand and walk without getting tired, but human is not design in that way." Then he also thought for a while and reply, "Oh....".

Well, I'm not sure whether my answer is logic but I'm just thinking, what will be the better answer for this question?