Fred Foobar leaves his house with his
shotgun, goes to a
costume shop and buys a
silly clown outfit, and proceeds to his local branch of the
Midland Bank.
Outside in the car he dons the outfit, wields his weapon and walks into the bank.
"Give me all ya fuckin' money and don't nobody move."
Unfortunately for Fred, no money is present in the bank! When he learns of this fact he quickly exits the building, but it's too late - the police have arrived and members of the armed response team arrest Fred.
Fred attempted to commit a serious crime - it can be proved beyond reasonable doubt that he intended to rob the bank. But it was impossible for him to do so - however, law states that what matters here is intent. Fred can in fact be charged for the full offence - and this is the impossibility rule.