It is hard.

Say someone posts a thread on 'what should I feed my certain animal'.
Then from animals the discussion goes like this;

feeding of animal----> what to feed them----> experiences with pets and diet----> problems arisen from improper care ----> thoughts on keeping pets in the home -----> animal cruelty

Has the discussion strayed from the OP? Yes.
But it is still 'on topic'.
Inevitable, especially with someone seeking info, the info is given and the thread dies.

But on here (which is a nice thing) threads continue.
What is annoying is, halfway through the part say, for example, about 'animal cruelty', someone makes a crazy point, how does that person defend it?

"that is not what the thread is about".

Which is infuriating, because discussion is supposed to evolve, not be suffocated.