There is a higher population of water molecules in the pure water in the container than there is in the sugar solution. I draw an analogy of an elevator which has 5 people inside. As the elevator stops at a floor, were it not for the reasoned manners of the people on the outside of the elevator trying to get in, none of the people on the inside would be able to get off because there are many more tring to get inside the elevator. Water molecules cannot reason. They simply abide by the brute physical laws.
More water molecules can get inside the tube then can get outside.
The direction of osmosis is always from the more concentrated side to the more dilute side of the membrane. As the sugar solution inside the tube becomes diluted, the water molecule population increases so that the osmosis toward the outside of the tube increases. Eventually just as many water molecules pass into the tube as pass out of it, and we have reached a state of dynamic equilibrium where the liquid level is in a steady state condition. Biological Applications Of Osmosis Osmosis displays itself in most biological systems that are cellular. The cell wall is a semi-permeable membrane.