Many people believe that making money on the stock markets is just a question of luck and that stock investing is quite similar to gamble at the casino. This is of course absolutely wrong. There are actually many rules to follow if you want to be successful when it comes to invest in stocks. In this post we are going to review one of these rules and we will review the other ones in future posts. In our mind, this is the most important one and the one that you should consider before starting investing. So, the first one is that you should never invest money that you cannot afford to lose. Why? Simply because even the best investors in the world lose money sometimes on the stock markets. It is impossible to constantly make profits on the stock markets and it is normal (and every investor should expect that to happen at one point) if your portfolio goes down in value from time to time. Does it mean that your strategy is bad and that you should change it? Does it mean that you are a bad investor? Not necessarily. It actually depends on many different things. First, what any investor should keep in mind is that when the markets in all go down, it is almost impossible for your portfolio to go up or even remain stable. It does not mean that this could not happen from time to time depending on some factors but as a general rule, when the markets as a whole are going south, your portfolio will likely go down too.