I love Foxtrot; it’s right up there with Calvin and Hobbes for me. I especially love the geeky humor, which I get, 99% of the time.
However, beyond the “Water” bit of today’s joke, I don’t get it. And of course, I am loathe to admit this because of Jason’s comment.
So someone help. What am I not smart enough to understand here? 
Look down.
M
2.71828 = e
r^2 = rr
(1/y)^-1= y
sqrt(x^2) = x
mass = force / acceleration
put it all together…
Doh.
I swear that thread was invisible a second ago.
But thanks for the explanation!
It says “Merry XMass”:
2.71828 = the constant e (approx)
r^2 = rr
(1/y)^-1 = y (taking something to the -1 “undoes” the reciprocal)
The square root of x^2 is x.
Force/Acceleration = Mass (a different way of looking at the more familiar F=ma equation.)
Since this is already asked and answered, many times, in another thread as pointed out by Loach, I’m gonna close this one.