Claiming to be wise, they became fools,  and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and creeping things. (ESV)
I remember my father saying to me when I was a child, "Use your head for more than a hat rack." Have you ever heard someone talk and wonder what they were actually using their head for? Paul is talking here about those men who have rejected the knowledge of God. They may claim to be wise, but rejection of God leads to utter foolishness.
In the Old Testament we read of all the Canaanite tribes who followed after various idols. These idols were often images of some kind of animal. They gave glory to that which represented the creation, rather than worshiping the Creator himself. We read of how men and women worshiped idols and wonder how they could be such fools. It starts by rejecting the knowledge of God.
In most of our "cultured" society we do not see people worshiping idols made of wood or stone. Yet, I would argue that many are still following various idols. Many of the atheists in our scientific community worship the universe itself. Rather than acknowledge the Creator they simply worship the creation. This is pure folly. As wonderful as the universe is, it is dwarfed by the glory of its creator.
Where else might we find idols? In Western culture we worship the idols of material wealth and our careers. You are only considered a success if you have achieved a certain level of income. When it comes to careers we worship at the idol of promotion and power. Even in the church we find that people are many times more concerned with idols of greed.
Use your head for more than a hat rack. Acknowledge God in every aspect of your life. Give him glory and allow him to lead your every decision. Rejecting the knowledge of God is the first step of folly.