ELI5: Why is it easier not to fall on a moving bike?

The way my father explained it to me was through conservation of momentum.

This sounds like a difficult concept, but all it really means is that a fast object (or a heavy object, but that isn't as crucial here) is going really fast, so it takes a lot of effort to stop it or move it, right?

This means that you're not likely to fall over, since you're heading in a straight line at a significant speed, but it also makes turning harder. It's easiest to take one's hands off the handlebars when you're going fast rather than slow for this reason (though unless you know what you're doing, don't try it).

The answer is much more complex than you think. In short, it is a combination of gyroscopic effect (spinning objects resist changes in direction), trail (the distance between the point of contact of the wheel with the ground and the intersection between the ground and the axis of steering), and the fact that small adjustments are required for large corrections at higher speeds.

it is mostly because of the tires spinning. "falling over" is essentially spinning towards the ground. but something that is spinning in one direction resists attempts to spin it in a perpendicular direction. if you invent a hover bike, with no wheels at all, it will be impossible to ride because you wouldn't be able to stop it from falling