So a mathematician, an engineer, and a physicist are out hunting
together. They spy a deer(*) in the woods.
The physicist calculates the velocity of the deer and the effect of
gravity on the bullet, aims his rifle and fires. Alas, he misses; the
bullet passes three feet behind the deer. The deer bolts some yards,
but comes to a halt, still within sight of the trio.
"Shame you missed," comments the engineer, "but of course with an
ordinary gun, one would expect that." He then levels his special
deer-hunting gun, which he rigged together from an ordinary rifle, a
sextant, a compass, a barometer, and a bunch of flashing lights which
don't do anything but impress onlookers, and fires. Alas, his bullet
passes three feet in front of the deer, who by this time wises up and
vanishes for good.
"Well," says the physicist, "your contraption didn't get it either."
"What do you mean?" pipes up the mathematician. "Between the two of
you, that was a perfect shot!"
(*) How they knew it was a deer:
The physicist observed that it behaved in a deer-like manner, so it
must be a deer.
The mathematician asked the physicist what it was, thereby reducing it
to a previously solved problem.
The engineer was in the woods to hunt deer, therefore it was a deer.