At the end of 'Titanic', Leonardo DiCaprio's Jack saves Kate Winslet's Rose by letting HER sit on the busted plank and wait for help while he floats next to her in the cold ocean...and eventually dies.

But people have pointed out that there was plenty of room on that plank for both Rose AND Jack.  So he could have just jumped on there with her, and Rose's heart wouldn't have had to go on alone.

A guy and a girl even proved it quite hilariously by putting an outline of the plank on the ground, and showing how many different ways they could comfortably fit within that outline.

So is this a glaring flaw in one of the most popular and successful movies of all time?  NO, according to director James Cameron because if both of them tried to float on that plank, it would have sunk.

He says, quote, "It's not a question of room;  it's a question of buoyancy.  Jack puts Rose on the raft, then he gets on the raft . . . He's not an idiot; he doesn't want to die...and then the raft sinks.

"So it's clear that there's really only enough buoyancy available for one person.  So, he makes a decision to let her be that person."

He adds that even if they COULD both balance on the raft, because of the buoyancy issue they'd be half-in and half-out of the water, and would have BOTH died of hypothermia.

He also said he thinks the guys from the show 'Mythbusters' are going to do a segment on this very conundrum and he wants to help them prove it once and for all.

Here's video.  Forward to the 4:18 mark.