Have you ever not known how to dispose of something so you said, "Eh, I'll just flush it down the toilet!" In those situations, have you ever considered that maybe that's not the best option?
There are actually very few things that are okay to put down the drain, so if you have to ask yourself, "Is it safe to put this down the drain?" the answer is probably, "No." So, while you probably know the stuff that is drain-friendly (human waste, greywater, etc.), let's discuss the items that are NOT safe to dispose of down your drain.