Imagine a cylinder that is full of water. A straw is attached to the bottom of the cylinder so that the water can slowly pour out. Attached to the straw is a second cylinder that lies perpendicular to the first. As water leaves the bottom of the first cylinder, it flows into the second cylinder that is lying sideways.

Now imagine that the water is pouring out the bottom of the fisrt cylinder and into the second sylinder.

If you were looking at the surface of the water in the second cylinder, from above, what shape would it take on when a quarter of the water has poured in?

If you were looking at the surface of the water in the second cylinder, from above, what shape would it take on when half of the water has poured in?

If you were looking at the surface of the water in the second cylinder, from above, what shape would it take on when three quarters of the water has poured in?

If you were looking at the surface of the water in the second cylinder, from above, what shape would it take on almost all of the water has poured in

The answer is that the surface of the water would take on the shape of a rectangle in all of these situations.

When a quarter of the water has poured in the rectangle would be small.

When half of the water has poured in the rectangle would it's largest size.

When three quarters of the water has poured in the rectangle would be small again.

When a almost all of has poured in the rectangle would be the smallest.