A fifth grader looked downcast

A fifth grader looked downcast, so her teacher asked, "What's the
problem, Carol? I hope it's not homework again."
"Well, uh, yes, it is." replied Carol. "I was stupid and made my
homework paper into a paper airplane."
"Carol, you're right, that wasn't a very bright thing to do," said the
teacher, "but this once I'll let you just unfold the paper and hand it
in."
"Oh, but that won't work," said Carol, looking even sadder. "You see,
the plane was hijacked."