You introduce three balls (red, yellow and green). You drop them one after another into the nested tube -- first the green ball, then the yellow ball, and finally the red ball. When the opaque tube is lifted, it is seen that the red ball is at the bottom inside the transparent tube, instead of being at top. The magician repeats this procedure once more, but the result is same -- the red ball appears at the bottom!
Lots of opportunities for humorous byplay!