As a youth I used to ride my bike everywhere. It kept me fit while I was young, and the general decline in my fitess that lasted until my mid-40s began with the winning of my driving licence. My bike took me everywhere I needed to go, these places not normally being farther than, say, ten miles away. It was my transport; to get anywhere I either rode my bike or walked. As a consequence, I know a lot about bike maintenance.
It looks like the the wife and I might be using our bikes on a upcoming holday, so I checked out our much-neglected off-road bikes. Mine was OK, but hers, having been lent to a young guest at our gîte recently had a flat back tyre, and the tyre itself had a number of bits on the tread that were worn away, where the toerag had skidded it as far as he could. So, off to get a new tyre, and suss out the leak(s)in the inner tube.
Mending a leak in an inner tube, is not difficult, in principle. You find the leak, clean the area around it using sandpaper, apply a very thin layer of rubber solution, and stick a patch on it. In practice it's harder. To find the leak, you inflate the tube and dunk it in water and see where the bubbles come from. Works like a charm. But as soon as the tube is out of the water, you can't see the leak any more. So you mark it with an "X" using a coloured wax crayon while it's under the water, except the crayon never puts the "X" exactly where the leak is.
Assuming you have a reasonably good fix on the leak, you take the tube out and clean the area around the leak with sandpaper, which makes it clean enough, but promptly erases the crayon mark. So you hope your leak is more or less in the centre of the cleaned bit, when you apply the rubber solution. You apply the smallest drop to the centre of the cleaned area, and then spread it around as thinly as you can with a finger. If you spread it properly, you get a nice circle of solution right around the leak. Now you apply the patch. It should stick.
Getting the tube back in place is easy, the tyre is difficult. You're not supposed to use tyre levers because this stretches the rim, so you have to push it on with your fingers. This can be done, with some effort, even if you have squashed one of your fingers the day before, fighting rock fairies.
Then you pump it up, only to discover that it deflates in half a day because the leak has has managed to be a millimetre away from the patch. It has happened.