You've probably seen the image to the right somewhere around the Internet. A lot of people have said that Jack was stupid and Rose was an ass when Jack didn't climb on top of the wooden door at the end of the movie, leading to his death. 

There's more than enough space for the both of them, the argument goes. However, the counterargument is that it's not about space in the raft, but rather a question of buoyancy. Check out Cameron's explanation in the video below (starting at about the 4:00 marker). The short explanation is that, Jack could've gotten on, but then both would have been half-in and half-out of the water and they both would've frozen to death.