Numbers love to do one thing more than anything else: pair up. Two by two, like passengers boarding a tiny boat. And it turns out the whole secret of odd and even hides in a single question — when everyone tries to grab a partner, does anybody get left standing alone?

Let's start with six jellybeans. Push them together in twos. One pair, two pairs, three pairs — and look, nobody is left over. Every jellybean has a buddy. When a number pairs up perfectly with no leftovers, we call it even.

Now try seven jellybeans. Pair them up the same way — and three pairs form, but one lonely jellybean sits at the end with no partner. That single leftover is the whole point. A number with one left over is odd.

So that's the rule, and it never breaks. Even means everyone pairs up. Odd means one is always left out, no matter how you shuffle them. You don't even need to count high — you just ask, "Will anyone be standing alone?"

Here's a shortcut so you never have to line up jellybeans again. Even numbers always end in 0, 2, 4, 6, or 8. Odd numbers always end in 1, 3, 5, 7, or 9. The very last digit tells the whole story — even a giant number like 3,847,562 is even, just because it ends in 2.

Even and odd also take turns, forever, like footsteps. Even, odd, even, odd — left foot, right foot, marching up the number line and never skipping. That's why if you know a number is even, the very next one simply has to be odd.

These two have funny habits when they get together. Add two evens, or two odds, and you always land on even. But mix one of each — one even, one odd — and you always get odd. The lonely leftover sneaks back in whenever exactly one odd shows up to the party.

And here's the neatest part of all: zero is even. It pairs up perfectly, because it has nothing to leave over — there's simply no lonely one. Zero might be the calmest, most relaxed even number of them all.

So the next time numbers climb aboard their little boat, you'll already know the ending. If they all find a partner, the boat is even and everyone's happy. If one is left waving from the dock, the boat is odd. Either way — there's always room for one more.