There is a general example of gambling in which the maximum winning amount is say $10,000 and you invest about $6000 in buying multiple tickets. In such a scenario the odds of winning the lottery are very low because of so many lottery buyers which poses high risk and the maximum return is $10000 only which gives net return of $4000, which is considerably low in comparison to the risk involved.

In another scenario of investing in stocks, suppose you invest in a company giving 15% annual returns while the minimum return in a risk free investment is 10%. The company you invested in is having its foundation very weak and facing a huge market competition. Hence you have a high risk of losing whole of your investment while the return is not as rewarding as the risk.

Making such an investment is a bad idea because there are high chances of making a loss and very minimal return which can even be earned in a comparatively safer investment option. A rational investor would choose to invest at minimum possible risk and maximum possible return or at a point where both these points are at trade-off.