Explain · Like I'm five version
Osmosis in terms of a busy Tube carriage
Osmosis is just like getting on a very crowded train when everybody wants to fit in.
Imagine you are standing on a platform at rush hour with a big bag of sweets. When the train doors slide open, the people inside are packed tight like sardines in a tin. If there is a big empty space at the other end of the carriage, the people near the doors will naturally squeeze past each other to fill that gap. They want to spread out because being squashed in one tiny corner is not very fun for anyone.
In science, water does the exact same thing to stay comfortable. When there is a lot of salty water on one side and plain water on the other, the water molecules move through a special gate. This gate is like the narrow doorway of the train carriage. The water wants to balance out so that both sides look the same. It keeps moving until the crowded side and the empty side have just the right amount of stuff.
Nature is always trying to make things neat and tidy. Just like a grumpy commuter who shuffles down the aisle to find a seat, water moves until it feels settled. It does not need a driver to tell it where to go or a map to follow. It just flows where it needs to be until everything is perfectly even and nobody is bumping into their neighbour anymore.
Where the analogy breaks downCommuters have free will and can choose to stay put, whereas water molecules move automatically and cannot decide to stay in the crowded section.