In these days of manic-depressive financial markets, it’s nice to be able to put your money into something that isn’t designed to make you rich, but that at least is pretty much guaranteed to always rise in value. And that, on occasion, can post a nice tax-free gain — most recently, 6.5 percent — overnight.

No, I’m not talking about some esoteric investment product with six levels of derivatives, pages of unreadable disclosures and fat management fees. I’m talking about . . . postage stamps.

Stamp investing, as usually defined, involves buying collector-quality issues, holding them for a while, then seeing if you can sell them for more than you paid. There’s big money to be made there — but it’s a game for the knowledgeable, not for amateurs.

By contrast, I’m talking about a small, simple investment that can be made by anyone in the United States with $10 to spend: Forever stamps. You know, the stamps that will always be good for the first ounce of a first-class mailing, regardless of how high the price has climbed since you bought them.

Forevers have risen in value by almost 20 percent, to their current 49 cents from the 41 cents that the Postal Service charged when it sold its first Forevers on April 12, 2007. More than a third of that value increase came Jan. 26, when the price of a first-class stamp rose to 49 cents from the previous 46 cents: the 6.5 percent overnight increase that I talked about.

One upon a time — before the Federal Reserve cut short-term interest rates to approximately zero — postal rates rose at a rate that about matched the yields on tax-exempt money-market funds: 2.5 to 3.5 percent a year. But since 2008, Forevers have pulled way ahead of money funds, which currently carry yields of 0.01 percent or so.

At my request, Vanguard calculated how much its major taxable and tax-free money funds earned between the first Forevers and Sunday’s postal rate rise. Answer: 7.3 and 5.6 percent, respectively, compared with 19.5 percent for the stamps. In case you’re interested, the stock market has done way better, even though it was near a cyclical peak when the stamps launched. The Standard & Poor’s 500-stock index fund (investor class) for Vanguard returned 45.3 percent during the period we’re looking at. But S&P returns don’t seem like an appropriate comparison to stamp returns. A fun one, though.

Unless you can figure out how to buy and quickly flip a million stamps or so — good luck with that — we’re not talking about Forevers as heavy-duty investments. Rather, we’re talking about them as a household-finance-type investment: saving a little money, conserving time by not having to wait in lines every year or so to buy 1-cent stamps, and getting some satisfaction from beating the system rather than having to use two 46-cent stamps to mail a 49-cent letter because you haven’t had the time or the inclination to update your postal portfolio.

Early this month, knowing that the increase was coming, I took advantage of a trip to the post office to mail a package by buying an extra 100-stamp Forever roll, which gave me two in inventory. The extra $3 I’ve saved isn’t all that much, in terms of money. But it’s fun. And I’m up 6.5 percent since the year started — which isn’t the case, alas, with my investment portfolio.

A minor caveat: The Postal Regulatory Commission told the Postal Service that 2 cents of the 3-cent increase this month is only temporary. The service is opposing any proposed rollback, and I doubt we’ll see a price drop. But if stamp prices somehow were cut, millions of Forever holders, including me, would feel cheated. And we would be reminded, once again, that in life, nothing is Forever.

Sloan is Fortune magazine’s senior editor at large.