I've once found some (something less than a gram), seriously. Nobody was around who it might belong to so I've just smoked it immediately. The law (well, a FAQ on the police website) says I should have taken it and given it up to the police but what if an officer would stop me for a check during my way and find it - wouldn't it be possession anyway? The purpose of possession is unverifiable and forbidding a person to consume any kind of substance is ridiculous.