HARD RIDDLE TO SOLEV WILL GIVE BRAINLIEST I have keys but no locks. I have a space but no room. You can enter, but can’t go outside. What am I?

He had fished in the river since ten years and he was a good fisherman. he know where to found the slim sliver chilwa fish, a big beautiful mahseer and the long moustache singhara. he know where the river was deep but where it is shallow; he knew which baits to use - which fish like worms and which liked gram. edit the above