A mutual fund, or an open-end fund, sells as many shares as investor demand requires. As money flows in, the fund grows. If money flows out of the fund the number of the fund's outstanding shares drops. Open-end funds are sometimes closed to new investors, but existing investors can still continue to invest money in the fund. In order to sell shares an investor usually sells the shares back to the fund. If an investor wishes to buy additional shares in a mutual fund, the investor must buy newly issued shares directly from the fund. (See Closed-end Funds)