Skip this paragraph if you know what a fuse box is. Not a breaker box, mind you. I am describing a similar but distinct entity. The function of the fuse box is the same as a breaker box-namely, it de-energizes an electrical circuit when you overload the circuit beyond the amperage it is rated for. Only, rather than tripping a breaker, a fuse has a metal filament within that melts, interrupting the circuit. Should this happen, you unscrew the fuse from the socket, dispose of it, and replace it with a new fuse rated at the correct amperage-hopefully, that is the same amperage of the fuse you just blew. There is a similar electrical device in your car you may be familiar with, and a few fuse boxes are still kicking around in a few older homes. You can still buy fuses in any hardware store.
For the matter of pennies. Well, supposing you blew a fuse, and you had no fuse to replace it with? The hardware store is closed, or you just don't want to go. The refrigerator is on the circuit you blew and chock full of perishables. Or the game is on TV. What to do? Some people would (and I imagine do) place a penny in the fuse socket and hold it in place with the blown fuse. The circuit is continuous once again, with the penny conducting electricity. Bad idea. Now there is no amperage protection. Now nothing stops anyone from overloading that circuit. So the circuit is overloaded, overheating, but the penny won't melt to shut it down, so what happens? The wiring melts elsewhere. Behind the walls. Starting an electrical fire.
"Penny in the fuse box" would be a great analogy for solutions that seem to work for now, but are not sustainable and will eventually result in disaster. Trouble is I usually have to explain what a fuse box is.