I always wrestle with the siphon thing, but I believe it works like this:
The water seeks it's own level and simply drains thru the siphon when it reaches the horizontal pipe that runs thru the dam. As it continues to rise and completely fills that same horizontal pipe, the weight of the exiting water (as it runs downhill out of the exit pipe behind the dam) begins the siphoning effect.
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Did I fail the exam?
