Everytime I am pouring bottled-water into a glass of syrup, I am thinking:
Water molecules are travelling in all directions in the water-body. While pouring bottled-water into a cup of syrup, the pouring water and the syrup come in contact and become one body. Can syrup molecules also travel back into the water bottle?
It is almost inevitable that some syrup molecules will have enough momentum to travel in the reverse direction, especially considering that you are putting energy into the syrup when you pour water into it.
This is because not all syrup molecules have the same velocity, there will always be a few with a higher than average velocity.
A Wikipedia article on probability distribution functions may be worth reading.
