Why do canals need locks?
Locks allow canals to go up and over hills. They sometimes are single locks, sometimes in flights of twenty or more. A lock is a bit like a big bathtub that you sail in to when it is nearly empty. You fill it up and then sail out the other end. This lifts the boat up to a higher canal level. Or it lowers the boat down to the lower level if you are letting water out of a full lock. To allow the boat to get in and out, and to keep the water in, there are gates at both ends of the lock. Sometimes there is one ‘single’ gate, sometimes two ‘double’ gates, for you and your crew to push open and closed. There are top and bottom gates and it is only possible to open a gate when the water levels on both sides are the same. So the top gate will only open when the lock is full of water, and the bottom gate only opens when the lock is (nearly) empty.
