Explain Quantum Computers like I'm five

Imagine I have a whole row of pennies, more pennies than you can see. Each one is laying heads up or tails up, right?

A regular computer is a big machine that rolls up and down the row of pennies flipping them over and looking at them to do all sorts of calculations and remember things its been told.

A quantum computer instead of simply turning pennies over and leaving them on the ground tries to flip them up into the air and keep them spinning as long as it can.

When we flip a penny in the air, is it heads or tails? We could say at one moment it's exactly heads; and the next it's exactly tails; sometimes it's exactly neither. Mathematically it turns out to be useful to describe it as "both". It doesn't mean the penny is literally both heads and tails, more trying to describe this time its flipping in the air; the penny will be either heads or tails at some time later, right now we don't know, it could be either, but we know how likely heads and tails are, and say its "both" (with 50% likelyhood of each).

So a quantum computer is a big machine that flips pennies in the air instead of keeping them on the ground like a regular computer. But what makes it actually useful and not just a coin flipping machine, is the quantum computer can blow air onto the flipping pennies while they are in the air to make them much more likely to land facing the answer we asked (like 99.99999% likely).

So for certain kinds of questions, a regular computer would have to roll down the whole row of pennies searching for the right answer, a quantum computer can do its coin flipping trick and get the answer much faster. But not all questions are like that. Regular computers are also much easier to build and we've been building them for awhile. Building a quantum computer that can flip a lot of coins correctly has been pretty hard, but scientists and engineers have been building better and better ones every year.

I always like the SMBC take.