This article is from the Puzzles FAQ, by Chris Cole email@example.com and Matthew Daly firstname.lastname@example.org with numerous contributions by others.
A sailor is in a sailboat on a river. The current is 3 knots with respect
to the land. The wind (air velocity) is zero, with respect to the
land. The sailor wants to proceed downriver as quickly as possible,
maximizing his downstream speed with respect to the land.
Should he raise the sail, or not?
Depends on the sail. If the boat is square-rigged, then not, since
raising the sail will simply increase the air resistance.
If the sailor has a fore-and-aft rig, then he should, since he can then
tack into the wind. (Imagine the boat in still water with a 3-knot head
If you are standing at the equator at sunrise, where must you point a laser
cannon to hit the Sun dead center? Assume that the Sun is stationary and
that the Earth's orbit around it is circular.
You aim it at the horizon. The sun is exactly in the place where it
appears to be. It is true that the sun wasn't on the horizon 8 minutes
ago (the specific number is 2 degrees), when it emitted the light you
are now seeing. However, "the sun wasn't on the horizon" doesn't mean
the sun moved; it means the horizon moved.