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Understanding the phrase “afford to lose” better
Well.... If you have alllll your money invested, and then there's a financial crisis, and there's a personal crisis at the same time (e.g. you lose your job) then you're in big trouble. You might not have enough money to cover your bills while you find a new job. You could lose your house, ruin your credit, or something icky like that.  Think 2008. Even if there's not a financial crisis, if the money is in a tax-sheltered retirement account then withdrawing it will incur ugly penalities. Now, after you've got an emergency fund established, things are different. If you could probably ride out six to twelve months with your general-purpose savings, then with the money you are investing for the long term (retirement) there's no reason you shouldn't invest 100% of the money in stocks. The difference is that you're not going to come back for that money in 6 months, you're going to come back for it in 40 years. As for retirement savings over the long term, though, I don't think it's a good idea to think of your money in those terms. If you ever lose 100% of your money on the stock market while you've invested in diversified instruments like S&P500 index funds, you're probably screwed one way or another because that represents the core industrial base of the US economy, and you'll have better things to worry about, like looking for a used shotgun. Myself, I prefer to give the suggestion "don't invest any money in stocks if you're going to need to take it out in the next 5 years or so" because you generally shouldn't be worried about a 100% loss of all the money in stocks your retirement accounts nearly so much as you should be worried about weathering large, medium-term setbacks, like the dot-com bubble crash and the 2008 financial crisis. I save the "don't invest money unless you can afford to lose it all" advice for highly speculative instruments like gold futures or social-media IPOs. Remember also that while you might lose a lot of your money on the stock market, your savings accounts and bonds will earn you pathetic amounts by comparison, which you will slowly lose to inflation. If you've had your money invested for decades then even during a crash you may still be coming out ahead relative to bonds.