A boy went to a Halloween party with a sheet over his head. Are you here as a ghost?

A boy went to a Halloween party with a sheet over his head.
"Are you here as a ghost?
" asked his friends.
"No," he replied, "I'm an unmade bed." Another boy wore a sheet over his head.
"Are you an unmade bed?
" asked his friends.
"No, I'm an undercover agent," he replied.