why does claudius send rozencratz and guilderstern to hamlet? what does hamlet tell them about his madness ? can we believe him?

Claudius sends Rozencratz and Guilderstern to find out what is making Hamlet act so bizarrely. Hamlet plays with his old friends exclaiming that he is only mad when the wind blows a certain direction, “ I am but mad north-north-west: when the wind is southerly I know a hawk from a handsaw." I think at this point at least, Hamlet is merely acting strange. He knows he is being spied on so he has some fun.