Unblocking drains
The blocked drain is the most common emergency and the most abused by aggressive home chemistry: acids and miracle products eat seals and turn a blockage into a leak.
What works at home
Plunger, very hot water with dish soap and a manual drain snake for the sink: if that does not clear it, the next step is mechanical, not chemical.
When it is the downpipe
If several drains clog at once or water comes up the shower when the sink empties, the blockage is in the shared stack: that belongs to the community and the building's service handles it.
Cheap prevention
Screens on every drain, no oil down the sink and a periodic hot-water flush prevent most call-outs. Paella oil never belonged in the drain.