If the universe is expanding and the distances of each galaxies get larger over time then why is our own galaxy (Milkyway) will collide with another galaxy (Andromeda) in the future?
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If you throw a ball into the air gravity will slow it down. Usually enough for the ball to stop and fall back to earth. But if you throw a ball fast enough gravity will not be able to slow it down fast enough to ever make it stop, the rate the ball is slowing is slower than the rate that the force of gravity shrinks, and it will fly away from earth. We call that escape velocity.
Ditto for galaxies. Most galaxies are moving away from us so fast that gravity will never be able to stop them relative to us and draw them back in. Andromeda is close enough where the speed is low enough where it will (or more correctly already has) reversed its direction and is heading back toward us.
This is a Hubble-type explanation which I would give to layperson. Expansion and dark energy may change the precise value.
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