Basic bike maintenance is something that everyone should learn. Even if it's just a spot of oil on the chain, a wipe over with a rag, or checking the air pressure in your tyres. Some other tasks are a little harder to tackle for the average punter, like changing disc brake pads, but one should always look/listen for the warning signs. Like a shreiking, scraping metallic sound, sparks flying from the wheel, and molten metal trying to fuse itself onto the rotor. This customer was juuuust a little late in his albeit correct prognosis that "I think the pads need changing". No shit, Sherlock.
At the top, a used but healthy pad. Below, the result of negligence, torture and abuse of a kind not seen since the days of Fred and Rose West.