Fat, oil and grease poured down the sink solidify as they cool down, and create wastewater blockages. There's a really interesting article today here about how fats and wet wipes create big problems in drains.
We should pour fats and oils not down the drain, but into a container, and then throw that in the bin. If you haven't got enough oil in the pan to justify using an old empty jar or similar, then soak it up with a paper towel and throw that in the bin.
What can go in the sink/down the loo - Pee, poo, paper, soap, water.
What should NOT - Vegie scraps, oil, fat, grease, wipes, pads and tampons, condoms, salad dressings, sauces, cream.