A dozen blocks are stacked on top of each other over the edge of the table seemingly defying gravity.

**What it shows:**

N objects of unit length can be stacked on top of each other so that the top object sticks out over the edge of the lecture bench by a distance equal to ^{1}

1/2 + 1/4 + 1/6 + 1/8 + ... + 1/(2N)

For N approaching ∞, the diverging infinite sum suggests that the top of the pile can stick out an infinite distance. In actuality the divergence is slow,^{ 2} and our more practical stack of a dozen 2 × 4 "blocks" can project 1.5 times the length of the 2 × 4 out over the edge of the bench. Only four "blocks" are needed for the top one to extend beyond the edge.

**How it works:**

The "blocks" are 2-ft long pieces of 2 × 4 lumber. They have been numbered and marked to expedite the presentation of the demo. Start by placing the bottom one on the lecture bench with a slight overhang and continue to build the stack with increasing overhangs until you end up with a pile looking something like this:

**Comments:**

As suggested by the figure, there are many ways to liven up this demonstration to make it great fun.

1 P. Johnson, Am J Phys **23**, 240 (1955) and L. Eisner, Am J Phys **27**, 121 (1959). R. Ehrlich has revived the demonstration in The Physics Teacher **23**, 489 (1985) and in his book *Turning the World Inside Out* (Princeton University Press, N.J., 1990).

2 Ehrlich points out that 1.5 × 10^{44} meter sticks are needed to get an extension of 10 meters!