Nothing spoils a nice drive like the creeping feeling of car sickness, but don't get too mad - this queasiness could actually be an indication that your brain is working as it should be... sort of.
Research has found that car sickness could be the result of your brain responding to what it thinks is a sudden bout of poisoning.
No, the guy in the passenger seat didn't put something in your coffee - scientists have suggested that when you're in a car, your brain is getting conflicting messages about your immediate environment, similar to when you've been poisoned. And we all know that throwing up is the easiest way to flush any neurotoxins or poisons out of your system.
So what's going on here, and why are our brains so confused?
Experts think that car sickness (or any kind of similar motion sickness) is brought on because humans have only recently started travelling in things like cars, buses, and boats, and our brains haven't fully adapted yet.
Despite the fact that we're travelling in a moving car, bus, or boat, the majority of our senses are still telling us that our bodies are stationery – and of course, your body is technically stationary when you're sitting in the back seat of a car.
At the same time, your brain also knows you're moving forward at a certain speed because of the balance sensors - little tubes of fluid - in your inner ear.
The liquid in these tubes is sloshing around, indicating that you're moving, but in reality you're sitting still. Your brain's getting some seriously mixed messages.
It's the job of the thalamus to piece together this info and figure out what's truly going on, but it often comes to the conclusion that poisons are to blame, which is why you'll sometimes have to stop at the side of the road to puke.
"As soon as the brain gets confused by anything like that, it says, oh, I don't know what to do, so just be sick, just in case," neuroscientist Dean Burnett from Cardiff University in the UK explained to Melissa Dahl at Science of Us. "And as a result, we get motion sickness because the brain's constantly worried about being poisoned."
Staring out of the window can actually help, because it reassures the brain that you are in fact moving and all is well. Reading a book or a map often makes matters worse, because it convinces the brain that you really are stationary and not speeding through space.
Being the driver helps too, because there's much more visual evidence available to the brain that you are genuinely on the move, and as an added bonus, you're the one in control of the movement. You're probably not being poisoned, in other words.
What scientists aren't certain about is why it affects some of us and not others, or why some people 'grow out' of car sickness. It's possibly just the luck of the evolutionary draw.
A 2013 study found that those with more 'body sway' – people whose bodies naturally move more often, even when stationary – were more likely to get sea sick. It might be that susceptible people just move differently in general, the study concluded.
Meanwhile, researchers continue to look for a cure to the ailment that's spoiled the start of many a family holiday. Based on what we know so far, listening to your favourite music can help some of the time, as can eating a light meal high in protein before the trip (it apparently helps to calm your stomach).
Asking the driver to pull over so you can throw up is never fun – but at least now you can explain some of the science behind your car sickness.
A version of this story was first published in August 2016.