What is cointegration?

Cointegration is when two things that seem to move all over the place actually stay connected in a special way.

Imagine you and your friend are playing on a seesaw. Sometimes you go up, sometimes down, it looks like you’re moving randomly. But if the seesaw stays balanced, there’s a hidden connection between how high you both go: when one goes up, the other comes down just enough to keep the balance.

Cointegration is like that balance. It happens when two things (like prices of related items) may wander around on their own, but they always come back together in a special way, like best friends who might bicker sometimes, but never stay mad for long.

Think of two kids counting marbles. One kid has a bag that keeps changing, sometimes more marbles, sometimes less. The other kid’s bag is also changing. But if the difference between their bags always stays about the same (say, 10 marbles), they’re cointegrated. Even though both are moving around, there's something keeping them close.

This hidden link helps us predict how they’ll behave together, like knowing that even if your friend jumps up on the seesaw, you can still keep it balanced!