Silly means:
1...weak-minded or lacking good sense; stupid or foolish: e.g., Your opinion is silly.
2...absurd; ridiculous; irrational: e.g., a silly idea.
Synonyms of silly are:
brainless, childish, crazy, empty-headed, idiotic, ignorant, illogical, immature, inappropriate, irrational, irresponsible, meaningless, pointless, ridiculous, senseless, simpleminded, stupid, unintelligent, unreasonable, unwise.
Debra - you wrote "i think silly means not smart, but not as strong as stupid."
I agree, however a smart dog or person can still act silly from time to time.
Although we usually think of "silly" as an innocent, non-harmful act or way of behaving.
For example:
1...The dog drank milk from a bowl and when he finished he had the white milk all over his face. He looked realy silly (comical).
2... I made a silly mistake when I lit my cigaret with a match, shook the match to extinguish the fire, then threw the cigaret on the ground. How silly of me!!!
3... Whe people translate a language word-for-word, the result is often a silly (non-sensical/nonsense)translation, silly enough to make you laugh.
4... My dog spent 30 minutes chasing (trying to catch) his tail. We laughed like crazy because the poor dog looked so silly (so ridiculous/so crazy/not smart) because if he could think better, he would realize that he could never catch his tail.
5... My friend drank two beers and started acting silly, dancing in a silly way, singing really badly, and making strange, funny faces. My friend was acting very silly, but it was fun.
I agree that stupid is much stronger than silly.
Stupid decisions can cause great harm.
Stupid action and decisions are more serious because we usually know better and we know that we should not or should never do the stupid action.
For example:
Driving a car at 150 kilometres per hour is stupid because we are not professional race car drivers, and if we may easily lose control of the car, with serious results.
We talk about silly decisions meaning to imply that the consequences of the silly action are not harmful.
I hope my answer does not seem silly - I'm sure it's not stupid.