If you try to walk step by step across a line, you usually only succeed if you can keep your balance.
Deep in your ear is a small ‘feeler’ for movements (the organ of balance). This balance organ senses when your head moves, for example when you turn your head to the left and right, but also when you just walk and “take your head” with you.
In order to properly maintain your balance, it is important that this balance organ works together with your eyes. This way you not only feel that you are moving, but you also see it. In the example of walking over a line you see, for example, when you almost step next to the line, but you also feel that you are deflecting a bit. A signal is then quickly sent to your brain to straighten your foot, and this way you keep your balance.
When you are looking at your phone in a car, your eyes will see the phone that you are holding and doesn’t see movement, but your balance organ feels you are moving forward in the car. The information from your eyes and balance organ that arrives in your brain then does not match and your brain no longer knows what to do. This can make you very sick, we call this motion sickness. It is often less if you look out the front window, or sideways at passing trees, because then you see that you are moving.