One of the best presents you can give a child is to fund their retirement. And it costs about the same as a modest car. The key to the gift is time. Having a long time to invest is critical but it costs you nothing but patience.

First let me explain how this works and then the four ways it could go wrong.

The initial step is to fund a Roth IRA. A child must have earned income but you can contribute an equivalent sum up to $5,500 a year to the child’s Roth. A Roth provides no deduction on the way in but if you hold it until age 59 1/2, there’s no tax on the way out. A $5,000 contribution to a Roth at age 20 and invested 100 percent in a diversified stock fund, earning the long-term return of stocks, would be worth $587,000 at age 70.

This sounds too easy so what could go wrong?

First, Congress could change the law so that the withdrawal would be taxed.

Secondly, inflation could eat up some or all of the returns. At the 3 percent inflation which has been the U.S. average in recent years, inflation would drive down the purchasing power by a third. Still, it would be a nice bundle to have in retirement.

Third, stocks could have disappointing returns. The longer one owns a diversified fund of stocks, the more likely that returns will be good but there are no guarantees in the stock market.

Finally, the child could mess things up in a variety of ways: invade the account early, change the investing approach or some other, unanticipated way.

Still, for the price of a regular car or less than a full semester at a good college, a parent could prepay a child’s entire retirement. An interesting idea and one I believe in strongly enough to try it.