Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

At what interest rate should debt be used as a tool?
This is a very interesting question. I'm going to attempt to answer it. Use debt to leverage investment. Historically, stock markets have returned 10% p.a., so today when interest rates are very low, and depending on which country you live in, you could theoretically borrow money at a very low interest rate and earn 10% p.a., pocketing the difference. This can be done through an ETF, mutual funds and other investment instruments. Make sure you have enough cash flow to cover the interest payments! Similar to the concept of acid ratio for companies, you should have slightly more than enough liquid funds to meet the monthly payments. Naturally, this strategy only works when interest rates are low. After that, you'll have to think of other ideas. However, IMO the Fed seems to be heading towards QE3 so we might be seeing a prolonged period of low interest rates, so borrowing seems like a sensible option now. Since the movements of interest rates are political in nature, monitoring this should be quite simple. It depends on you. Since interest rates are the opportunity cost of spending money, the lower the interest rates, the lower the opportunity costs of using money now and repaying it later. Interest rates are a market mechanism so that people who prefer to spend later can lend to people who prefer to spend now for the price of interest. *Disclaimer: Historically stocks have returned 10% p.a., but that doesn't mean this trend will continue indefinitely as we have seen fixed income outperform stocks in the recent past.