Earlier quoted context omitted.
Or even throw it in a Series I risk-free savings bond paying 8%. [edit] I'm not an expert but explained why I wouldn't do that personally in a peer to your reply.
Your comment, though sarcastic, underlines some points I thought about myself and I would like to discuss it! We are both well aware that there is no risk-free investment. Governments bonds that are considered less risky that ETFs usually return 1-2% per year. The ETF I'm talking about maps a market index covering a big chunk of the US market or several stable international markets. They are still a risk, your house…
Your mortgage is a liability, not an asset.