Every single person on the plane died except for 2. How is that possible?
It said all the single people died; the 2 were a couple. That's how it was possible.
Listen up, buttercup. The joke plays on the double meaning of "single." You see, most folks would think "single" means one person, but in this case, it means "unmarried." The couple were still alive because they weren't single, get it? Don't strain your brain too much, I know it's tough for you.