>>1309230let me use a picture
the blue is the tyre
the salmon is the innertube
the red dot is the bead of the tyre
see how the bead of the tyre sits on a hook on the rim?
And see how the innertube sits further into the rim, into the well of the rim?
If you deflate the tube all the way, you can work the bead of the tire off the hook, into the well of the rim, with your hands.
The well of the rim is a smaller circle than the hook of the rim. So the tire bead will fit more loosely on it, and it will be easy to push it over the edge of the rim, at one point.
Get it?
