First cut the cardboard into two strips. The first strip should reach about half way across the box, the second strip should slightly overlap the first strip. Tape the first strip so that it is stationary within the box.
Tape the second strip so that it swings on a hinge.
Now place the marble inside of the box and tilt the box so that the marble pushes open the second flap.
Then tilt the box back so that the marble presses the second flap down to the first flap and the marble cannot pass through again.
Now picture that the marble is our blood. This is how the valves in our veins allow blood to flow one way only and carry the blood throughout our bodies.
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