Try this with your class next time they find it hard to grasp the concept of osmosis. 
 
Place chairs or stools in the middle of the room, with space between each enough for a student to pass through. Have most students standing on one side of the room, with fewer on the other side. Ask all students to move around randomly, bumping (gently) each other as necessary in a way that they would all have the maximum amount of space possible. Question them about their movement experience on either side. Relate to the process of osmosis and how it contributes to movement of substances within the body. 

Chairs/Stools = Selectively Permeable Cell Membrane 
Students = Water molecules 

This is good as an engage activity to the topic.