Understand the principles
of hydrodynamics, i.e. water
flows downhill
A lock works because water wants to go downhill. By controlling the flow of water with two lock gates and with a means to let water in or out, a boat can go either up or down. When a boat enters a lock, the gate closest to the boat must be open and the gate farthest away must be closed. Water must then be let out if the boat is going down or let in if the boat is going uphill. Then the position of the gates is reversed.
Even in the UK, water is not an inexhaustible commodity and occasional droughts have forced the closure of some canals. It’s important to close gates and paddles to prevent water loss.
A lock also works because there is an “inexhaustible” supply of water feeding the canal at its highest points. That’s one reason canals usually follow river courses—to have a steady water source. You’ll also find pumping stations and reservoirs near these high points. (The other reason canals follow rivers is that rivers have already excavated a nearly level path the canal can use. Later railroads would take advantage of the same course the canals took, sometimes filling the canal for railway beds.)
When going downhill, a lock is in your favor if the pound or lock chamber is already filled. When going uphill, a lock is in your favor if the lock chamber is empty.