A salmon begins its life in a shallow, swift-moving stream.
After feeding for a few weeks, it begins a long journey. It
follows the current down to another stream, perhaps to
another, then out to a river. It eventually swims out into
the ocean, where it roams a vast area, sometimes traveling
thousands of miles from its birthplace.
If that weren't remarkable enough, the salmon will someday
return home, back to the river mouth from which it emerged,
back up the same waterways it descended, back to the very
stream where its life began.
It's an amazing journey—one that has caused more than
one biologist to scratch her head. But experiments have
shown that the fish actually smell their way to their
nesting sites. Odors from the stream—perhaps minerals
and organic substances washing in from the surrounding
land—become more intense as a fish swims upstream and
help it decide which route to take whenever it encounters a
fork.
In this activity you are a salmon trying to find its way
home. Since your human sense of smell is not nearly as acute
as your fishy relative's (and since the technology that
spits out odiferous molecules from a computer peripheral has
yet to be developed), you'll have to rely on the "smells
like home" meter within the activity to help guide you
along.