The required components are oxygen, fuel, and some means of ignition. Except in unusual circumstances (under water, in a vacuum, etc), oxygen is pretty much always available. Fuel and the means of igniting it are another matter.
All the fuel in the world won't help if there's nothing to spark it with. A spark is useless if there is no fuel available or if the fuel is wet or otherwise ill-suited to sustain combustion.
Getting kids to learn is a lot like encouraging combustion. If the fuel is wet, you will have a hard time getting it ignited, and keeping it burning is going to be a chore that requires constant effort. If there is little or insufficient fuel, the spark will quickly die.
If we use arbitrary or contrived criteria in choosing our spark and fuel, we are likely to have a difficult time sustaining combustion. But quite the opposite is true if we make the effort to select fuel and means of ignition carefully. If we do this, we will scarcely be able to prevent learning from happening.
Experience with real fire provides us with a rich set of metaphors. A huge log, even if it is completely dry, will not ignite from a single match. Lumber that is tightly packed together is much harder to light and keep going than the same lumber spaced apart from each other. Even dead leaves and small twigs won't light if they are wet. On a large, hot fire, small amounts of fuel will be consumed very quickly.
Much of what we present in traditional educational practice amounts to large masses of wet leaves, huge, dry logs, many logs packed tightly together, and sparks with little or no accompanying fuel. By looking at what naturally interests kids, and finding appropriately prepared fuel to keep that interest burning, we can build fires that burn with spectacular heat and light, not to mention ease and efficiency.
When the conditions are just right, you can have spontaneous combustion--not even an external spark needed. And it is good to note that perfect conditions for sustaining combustion occur in nature as well.
When we see an educational effort failing, we would do well to ask ourselves whether the spark is the right type, and whether there is appropriate fuel present. Chances are that we'll be able to find a problem in one area or the other, and move on to a, um, brighter future.