A person may not be stupid, but still do silly things or be acting silly. In English, and in Western culture in general, there is a difference between saying someone is acting stupidly versus saying that they are stupid. The first implies a temporary lapse in judgment. Everyone, no matter how intelligent, does something stupid from time to time. The second is a judgement on the person themselves. It implies that they not just acting in a stupid manner, but that stupidity is part of their very core, something that they can't improve on or escape. The second judgement is considered to be much more of an insult than the first.

Hello Claire, In informal expressions "stupid" doesn't have to mean always lacking intelligence or unable to think clearly ,it could also express being bored of something or someone " your stupid stuff!!" When you say something " silly" or ask a silly question sometimes it is not really out of foolishness, but with an intention of being funny or making people laugh. When you tell someone "Don't act so silly" you rather sympathize with their attitude!. If I told you that the difference between them both is "lly and tupd" then I am being silly ,but if I told you that there is no difference at all then I am rather stupid !

As Forrest Gump said: "Stupid is as stupid does" ....I'm sorry, i can't stop to do silly things....

Silly - showing a lack of thought, understanding or judgement, stupid and embarrassing, especially in a way that is more typical of a child than an adult; not practical Stupid - showing a lack of thought or good judgement; slow to learn or understand things, not clever or intelligent. If you call sb stupid, you are telling them, usually in a joking way, that you think they are not being very intelligent.

Silly is an adjective that means foolish Stephen painted his nose red, he is very silly. Painting your nose is a silly thing to do. Stupid is an adjective that means unintelligent Stephen tried to open the car door with a fork, he is very stupid. Trying to open the car door with a fork is a stupid thing to do.