If I open a big tub of yogurt, eat from it, and put it back in the fridge. When I open the tub the next morning for some more yogurt, there is a layer of water / watery yogurt floating on top of the tub. I'm told all I need to do is 'stir' the water back in, but why does this happen?

Also, why does the yogurt not go watery before I open the tub? If I keep the tub in my fridge for a week before opening it, the first time I open it the yogurt is fine.

Lastely, is there a way to prevent the yogurt from going watery? I find it rather off putting.