He shared an interesting metaphor on building knowledge that resonated with me, not the least of which because it was geological!
He suggested that building up one's base knowledge is like creating mental bedrock. Bedrock is created by layers of sand and sediment being superimposed on each other over time, usually underwater. When the sand is initially laid down, it's still free to move around - some of it could be swept away by turbulent water, or disturbed by a fish swimming by. We can imagine this sand as new bits of knowledge when we first come across them.
But over time, as we learn more things and use those bits of knowledge in different ways (connecting them to other pieces of knowledge in other disciplines, say), more sand gets laid down on top of the previous layers. Only after lots of sand gets added, and we have the pressure of the water above it (lots of practice or use of that original knowledge), do those formative layers of sand harden and become bedrock - rock so hard that it is nearly impossible to chip away... a far cry from the easily-moved sand when it was originally laid down.
The top layers, my colleague argued, were the new knowledge we are continually gaining every day. It might sit in place in our brain for a bit, or it might get swept away if we don't use it regularly. But all that new knowledge is held up by the mental bedrock underneath it: knowledge that now comes to us quickly because of years of use and experience.
I like the idea that building this bedrock of knowledge takes time and effort. You can't just learn something once to have it stick - it has to be revisited, reconsidered, and re-applied. Mastering something new isn't always easy, but if we can persevere and work through the initial learning curve, what we end up with becomes, if I may say so, rock solid.
The question then becomes: how thick should our base layer of bedrock be? How can we, as teachers, continually reinforce what students have previously learned so that one day it comes easily to them? In a world where all sorts of basic knowledge is at everyone's fingertips, how can we make sure our students have a good foundation upon which to build future masteries?