<prompt> Human: Hi personalfinance,I have a money market account with WAY too much money in it doing nothing. I have been waiting for a downturn in the market to invest, but aside from BREXIT which I was out of town for, the market has seemed pretty steady for 6 months. To be honest, the market scares me a little bit.I am not rich by any means but am very fortunate. I currently owe 202,000 on my house which is worth about 550,000 and have a 4.25 fixed rate. I have 65,000 in a money market account, and am looking to invest around 50,000 of that. I also have an emergency savings account with around 15,000 and my daughters college fund is invested in not included in any of those accounts. Together my wife and I bring in around 120,000yr and have zero debt thanks to mostly full ride college scholarships.My question is: 4.25 seems like a decent rate of return on investment, if I just dump the 50,000 into my mortgage isnt that basically a 4.25 return automatically without having to worry about the market?My wife and I are both science people, not business people. Sorry if this is a stupid question.EDIT: Thank you for all the great input. I had basically read through and decided that I was going to refi to a 15 year mortgage, I went to my online bank website and realized Im an idiot. My current mortgage is actually only 3.25, it was my construction perm loan that was 4.25, which has been closed. This muddies the waters a bit and I think Im going to do the 5k investment every few weeks until I invest all 50k.Assistant: Waiting for a downturn in the market to invest is the definition of timing the market, which is something you should not do. You can pay down your mortgage and get an instant 4.25 on your returns. This is true. However, if you can invest that money and beat 4.25, you would be wiser to put that money there. The USUK stock market has an average return of about 9, historically.Human: I know this is bad... but I got burned when I did my daughters college fund, and Im still a little sour about it. I invested February 2015, and the return has basically been 0 in 18 months.  <response>: 18 months is not enough time to judge a stock market investment. 