On my desk is a tube of lip balm. If I hold it between my fingers, ready to drop it, how do I know it will fall?
It seems like a very simple question, but I don't think it is. I can dumb it down much, much further.
The most fundamental level of knowledge is the present sensory experience. I know THIS more than anything else.
We don't need to know how we sense reality, only that we sense reality.
We have a memory, which, on its own, is not enough to consider knowledge. But then what validates our memories?
Our capacity for reason: our intelligence.
The simplest question I suppose you could ask, without worrying about the self-evident sensory experience, is how we know there will be a future.
How do we know there will be a future?
First comes the conception of time: Our memory records and retrieves sensory information in order of occurrence.
We validate our memory of half a second ago by acknowledging it's consistency with our present sensory experience. We see that reality is here, and remember that it was here, and so we conclude that in half a second from now it will still be here.
Can we continue this process of reasoning to eventually explain how we can know something like gravity?