Picture a glass full of water, right up to the brim. If you try to pour more water in, what happens? It all spills over and nothing new goes in! Our heads work kind of the same way: when we think we already know everything, there's no room left to learn new things.
The smartest people in the world aren't the ones who say “I already know.” They're the ones who love asking questions, because they discovered a secret: nobody knows everything. There's always a little more to learn.
And there's Someone who knows absolutely everything: God. That's why the smartest thing we can do is ask Him to teach us. Even Jesus, who is the wisest of all, was the most humble, and He said: learn from me. Whoever has a humble heart, like a glass with room left, keeps filling up with real wisdom.
A head that thinks it already knows everything has no room to learn.