If you imagine each mug to be filled with 100 balls representing droplets, then it is easier. If you take, let's say 25 droplets of tea into the milk, you have in one mug, 75 drops of tea and in the other 100 drops of milk and 25 drops of tea. Then, take 25 drops back, (staying equal to 25 drops, of course) let's say 5 drops of tea and 20 drops of milk. You then have 80 drops of tea and 20 drops of milk in one mug, and 80 drops of milk and 20 drops of tea in the other.