If we divide a cube with an edge of $1 \mathrm{~m}$ into small cubes with an edge of $1 \mathrm{~mm}$, what will be the height of a column formed by all the small cubes, placed one on top of the other?
(a) $1 \mathrm{~m}$
(b) $1 \mathrm{~km}$
(c) $10 \mathrm{~km}$
(d) $100 \mathrm{~km}$
(e) $1000 \mathrm{~km}$
The correct option is (e).

Converting meters to millimeters, we have $1 \mathrm{~m}=1000 \mathrm{~mm}$. Thus, the cube was divided into $1000 \times 1000 \times 1000=10^{9}$ small cubes, each with an edge of $1 \mathrm{~mm}$. Stacking the $10^{9}$ small cubes one on top of the other, we obtain a column $10^{9} \mathrm{~mm}=1000 \mathrm{~km}$ high.