We've all been in the situation before - choosing the free option when booking seats on flights, only to be seated a mile away from your friends or family.
So, what's the harm in moving next to them if there's a free seat on your travels?
It turns out that empty flights aren't an invitation to move around, and it's for the safety of everyone on board.
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You may be sceptical about this, especially since many budget airlines charge you to choose seats, but science is involved in this safety measure.
It's something that a lot of passengers may not know about, but the topic has unsurprisingly done the rounds on social media.
Taking to Reddit, one user asked: "My flight just announced that it will be pretty empty, and that it is important for everyone to sit in their assigned seats to keep the weight balanced.
"What would happen if everyone, on a full flight, moved to one side of the plane?"
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In reply to the question, a 'former aircraft load-planner' said that while it would depend on the size of the plane itself, moving weight around 'is always something of concern.'
They went on: "Moving from one side of the aircraft to the other wouldn't have nearly as much of an effect as moving everyone from the front or to the rear.
"Think about the shape of a plane as a big lever with the fulcrum running sideways through the wings also down the middle of the plane."
Apparently, moving from left to right or vice versa wouldn't do too much due to how the plane is laid out.
In the most extreme cases, such as the National Air cargo flight 102 in Afghanistan back in 2013, the aircraft can crash and kill those onboard.
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However, it should be noted that there was a heavy load of armoured vehicles on board, with unsecured vehicles sliding to the back of the aircraft, resulting in a loss of control - as shown in the harrowing footage on YouTube.
Luckily, the user said that on a standard Boeing 747, if people were to all move to the back of the plane, the pilot could simply move fuel or add trim - adjusting the aerodynamic forces on the control surfaces.
Another user pointed out Ryanair's regular announcement when you're boarding, where they point out that seating has been allocated in line with 'balance rules' and 'the weight of the aircraft', though they were sceptical.
Pointing out that Ryanair flights are mostly full anyway, the user thought that it may be the airline up to some tricks but later agreed the rules are about safety.
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They initially said: "Clearly they don't take it into account when paying for a specific seat (let alone the different weight of passengers and their luggage)."
But after being educated, they admitted: "When the plane is quite empty there are times when people are asked to move.
"So in that sense the announcement is not BS."
Joshua Nair