The investments you choose and the returns associated with them should be in line with your risk tolerance. Decide whether your investment priority is growth, income, or conservation of principle? In general, your priority depends on your age and position in life. If you are fresh out of college, accumulating wealth is your priority. As you get older and are in the prime of your earning years at work, your priorities are mixed. The focus begins to change and you want mostly growth but conservation of principle starts to become important. By the time retirement becomes affordable and work becomes optional, growth is less of a priority and conservation of principle is your main concern. Once retired, growth is still important so that you keep up with inflation during retirement, but preservation of principle and the amount of income provided become your investment priority.

If you are playing the market just for fun (with the potential for big gains) such as buying dot com stocks, that's great. But it would not be wise if you are investing this month's mortgage payment and counting on your gains to be there. In the short term this strategy is horrible when you need the money that you are investing. You can see how it would be prudent to put this money somewhere safe until you need to write the mortgage check - like your checking account or a short term CD. For the long term however, keeping your money safe in your checking account would be a mistake.  It is important for you to identify your time horizon (when will you need this money).