I cut a panel out of the top of each milk carton. Children could then pour water into any milk carton of their choosing. Water emptied out of each carton into the next through each of their original spouts.
Axiom #6 on the right hand column of this blog asserts that children will stop or redirect the flow of any medium in the table. In the picture below, the child nicely illustrates this axiom because he blocked the flow of water from the cascade with his hand. In addition, he poured enough water into the bottom milk carton to make it overflow.