4. Investments and Other Assets

In 2016, we purchased 10% of the outstanding common stock and associated voting rights of a privately held Colorado corporation for $10.0 million. We are accounting for this investment using the cost method. As part of our initial investment, we have the right, but not the obligation, to purchase additional shares. If we elect not to utilize our right to purchase additional shares or transfer these rights to another party by certain deadlines, we may be required to surrender and forfeit our existing stock ownership.

Other assets consist of the following as of December 31: