The most important skill a young rider needs to learn to ride a bike is balance. Everyone who knows how to ride a bike necessarily knows how to balance it. But can you articulate exactly what you do to keep a bike balanced? For the purpose of this article, we're going to focus on one big question:
How does a rider balance a bike?
Balancing a Broom
Riding a bicycle bears resemblance to balancing a broom vertically on your hand. When the broom is perfectly vertical, the broom will stay balanced on your hand. But if the broom moved even slightly off vertical, the top of the broom will begin to tip and fall towards the ground. You can keep the broom balanced by moving your hand in the direction in which the broom is falling, bringing the broom back into a vertical position. Keeping the broom balanced is a game of constantly observing the top of the broom lean over, and moving your hand under the broom...over and over and over again....for as long as you long to play the broom balancing game. In sciency terms, you are keeping the support of the broom (the part of the broom that touches your hand) directly underneath the center of mass of the broom.
There are also effects that assist the rider, including the gyroscopic effect of the wheels and the steering torque effect of rake. But, neither of these effects is directly responsible for keeping the bike upright. And since we're interested on what it is that the rider is doing to keep the bike balanced, we're going to ignore these effects.
We're also going to ignore centrifugal force. Centrifugal force comes into play when a rider is turning. It's why rider motorcycle racers can lean way over in a turn and not fall over. Centrifugal force shifts the place that the wheels must be placed in order to keep the bike balanced. It also complicates this discussion with a force that is not necessary to keep a bicycle balanced, so we are going to ignore it.
A rider balances a bike by steering. More accurately, a rider balances a bike by using steering to constantly keep the wheels centered under the center of mass of the bike. If that makes perfect sense to you, you can stop reading - you have graduated. If that only kind of makes sense to you, or if you are totally confused, no worries. We're going to break this down nice an easy. And if you are still with me, good news: pictures are coming up
Unless the mass of the bike is perfectly centered over the wheels (hint: the mass is essentially never perfectly balanced) gravity will always be leaning the bike towards the ground and the rider must constantly be counteracting these leans by turning the bike and generating centrifugal force in the opposite direction of the lean.
Illustration of a Rider Using Steering to Keep Wheels and Center of Mass Vertically Aligned
Starting Out: We're going to start from a point of balance. This is when the support (wheels) are directly under the center of mass.
Getting Imbalanced: The bicycle begins to tip in one direction. This moves the center of mass out of vertical alignment with the support.
Correcting: The rider steers in the direction of the lean. This moves the support (the wheels) under the center of mass.
What's the takeaway from this?
When a person rides a bike, the rider is constantly correcting for lean -- constantly steering to move the wheels under the center of mass. The rider's brain is making very rapid and precise adjustments without the rider consciously thinking about it. The ability to make these constant precise adjustments to maintain balance is the essential skill that young riders need to learn.
Once you learn how to balance a bike, it happens without thinking. It's as intuitive as walking. Because balancing a bike feels so intuitive, sometimes we forget how frustrating it was to learn the skill in the first place. If we (1) empathize with the frustration of learners and (2) understand the mechanics of bike balance, then we can start to make changes to the way that we teach kids to ride that will make the process less scary, more fun and easier.
 Rake serves to help the bike self correct since it causes the bike to turn in the direction of lean. Similarly, the gyroscopic effect of the front wheels exerts a force that turns the front wheel towards lean. So rake and gyroscopic effect both assist the rider in turning into lean and generating centrifugal force that counteracts gravity. Neither rake nor gyroscopic effect alone keep the bike upright. And bikes can be ridden with zero rake and with gyroscopic effect canceled out. But bicycles cannot be ridden with steering locked. If you want to learn more about this I recommend reading Bicycling Science (MIT Press), by David Gordon Wilson. Or we can have a nice internet debate about this in the comment section - just keep it civil, this is a kid's bike company.