Re: How can I find out why running water + knife blade removes onion smell?

Well, I still haven't been cooking with onions yet, but when I
do, I'll try your idea as best I can.

First, you seem to have made an interesting observation.  I'm not entirely
sure I understand what you mean, whether you are talking about removing
the smell from your hands or the knife or whatever.

But once you've made an observation you need to do an experiment.  First
you need a control to compare the experiment to.  I presume that would be
to have no running water.  

Then you need to measure the effect.  That's hard with smell.  In
order to measure it objectively you can't have your feelings interfere
with how you perceive the smell yourself.  The best way to do that is to
have someone else who is "blindfolded" and who doesn't know if the knife
blade or whatever has been under the cold running water.  Only if their
findings consistently agree with what you think should be happening, will I
believe that something reproducible and scientific is happening.

Part Two: 

Odor is when, as you breathe in, the air is pulled through the nose.
Special sensory cells (of which there are many different types) react to
different molecules in the air when these molecules hit the surface of
these cells.  Then they "react" to a new molecule by sending a message
through nerve to your brain which makes your brain feel and think "I smell
something".  Although air doesn't have a smell by itself, lots of other
molecules trigger a "smell" message.  Onions have small oily sulfur
containing chemicals in them which are very strong at making a smell
response.

When you cut open an onion, the cells of the onion a sliced open and its
insides are exposed.  The whole thing is so rough (from the cells point of
view) that you can see how lots of these molecules are broadcast into the
air where they float around waiting to pulled up into your nose.  And then
you'll say, "I smell an onion".  Some of these molecules actually irritate
the moist cells of you nose and eyes, which if you get enough of them will
make you cry!

These oily smelly onion molecules can dissolve in water.  So running water
can wash them away and then you won't smell them as much.  Also, water
just rushing by in air will pull the odor molecules from out of the air
which will reduce the smell.

Power plants burning coal also produce different types of sulfur
containing molecules which are bad to breathe and cause pollution.  So the
exhaust from a power plant can sent through giant chambers with running
water in them to "scrub" the air clean.  This "scrubbing" soaks up all
those bad pollution molecules.  You may be using your kitchen sink as an
onion smell scrubber.

If the onion touches your skin, the irritating molecules can pass
through your skin and travel through your blood stream and then you'll
feel their effect on your eyes.  If your hand is under water or under
running water, then these molecules will tend to be carried away by the 
water and won't be able to get into your skin.  I don't know what you'd
call it except that the molecules are being "diluted out" or "carried
away" by water.

Again, I'm not exactly sure what you're talking about, but there are some
of the things which may be going on.