[This is something I know I've always wondered about]

Well, Whadya Know

Why is it easier to tear an article from a newspaper from top
to bottom than from side to side?

In order to print the news, you need newsprint, the wood pulp paper product on
which the opin . . . uh facts go. That pulp is broken down
into many small fibers that tumble onto a conveyor belt.
This process naturally aligns them in the direction the belt
is moving, a direction they retain when formed into sheets
and then large rolls of newsprint: straight up and down.

Thus when you clip an article from top to bottom you go with
the grain and achieve a smooth cut; tear it out from the side
and you are ripping against the grain and risk losing part of
the article.

[source: Ever Wonder Why? By Douglas B. Smith]