Video narration
Why does the same cell swell in one solution and shrink in another? In a crowded room, people tend to pass through a doorway from the packed side toward the emptier side. At the doorway, particles spread from more crowded to less crowded: diffusion is passive transport, following that concentration difference without energy. Moving against that flow takes effort: active transport uses energy. Osmosis is water crossing a selectively permeable membrane, with net movement toward the side with more effective solute. Water moves both ways; net is what matters. The key is comparing solute inside and out: that predicts net water movement and cell size. At the doorway, hypotonic means less solute outside, so net water enters and the cell swells; hypertonic means more, so water leaves and it shrinks; isotonic means equal, so there's no net flow and size stays about the same.