I once heard of this saying too when I was little. I am sure watching it is very bad to eyes. It is easy to understand. Eye works like camera. And pupil is the aperture. It controls the light entering retina. When the environment is dark and light is dim, pupil will dilate to let in more light so that it is easier for us to see. Yet there is a special part in eye, which needs to take a while to be active. This is what we said we need eyes get used to the dark environment. When this part is active, we can feel easier to see clear in dark. But if we watching the lightening, our pupil is still dilated, it can't adjust in time, and then too much strong light will get into retina, which could damage retina. It is better not to watching lightening just like you can't stare at the sun directly. if you keep doing this, sooner or later, you might lose your vision. Eyes are very delicate organ, so we need to protect them carefully.
