Maybe put money into a commodities index fund . . . anything that smells like metals, oil or foodstuffs will keep on soaring.

Commodity prices have been doing just that lately.

The Goldman Sachs Commodity Index, which tracks prices for 24 energy, metals and farm products, last week rose 1.8%. Everyone from hedge funds to Uncle Phil's dentist is chasing after more easy money.

Wall Street, of course, eagerly obliges the fun-seekers, brokering trades, talking up the rallies and creating new speculative vehicles.

Look at the popularity of the Pimco Commodity RealReturn Strategy Fund that invests in commodity derivatives (don't ask). Its assets are now $12bn. On 10 April, people began trading shares in US Oil Fund, which tries to mimic movements of contracts for future deliveries of West Texas intermediate light crude oil.

This must mean that the markets are peaking. For certain, the flood of speculative money has unhinged them from reality.

While global economic growth has increased the demand for commodities . . .eroding inventories of copper and sugar . . . speculators have pushed prices well beyond what would be set through normal negotiations between producers and customers.

It's easy to see what's sucking folks in. The Goldman Sachs Commodity Index is up 13% so far in 2006. By comparison, the benchmark Standard & Poor's 500 Index of stocks is up 4.8% this year.

And in the same time, US Treasury 10-year securities have declined, the yield rising to 4.98% from 4.39%.

Copper, which is used in wire and plumbing, traded at a record $6,500 a tonne for future delivery in London last week. Copper-miner Phelps Dodge, based in Arizona, has declared about $1.3bn in special dividends in the past six months and says it has another $700m available for dividends or stock buybacks.

Crude oil futures also hit an all-time high in New York at $71.60 a barrel. Sugar, used in food and as a gasoline substitute in Brazil, is frothy too.

Even zinc, used to coat steel to keep it from corroding, set a record last week. And as zinc goes, so goes lead, which is used in car batteries.

Speculators have forgotten, if they ever knew, that commodities are notoriously volatile. It's simple. When prices soar, miners mine more, factories make more and farmers plant more. Soon supply outruns demand and prices drop, often disastrously. Check any economics textbook.

What if simultaneously extraordinary economic growth rates in China and India slow to just above average? That would reduce demand for energy just at a time when, as analysts for Paris-based Societe Generale pointed out last week, Saudi Arabia and other members of the Organisation of Petroleum Exporting Countries rev up crude oil production.

And what if Americans stop buying gasoline-thirsty sport utility vehicles at the same time? Glowing predictions for sugar-based ethanol might then look wide of the mark.

More sluggish economies would also crimp demand for steel and aluminum. A slowdown in US home-building can't bode well for sales of copper wire and plumbing.

The world won't fall apart when commodity prices collapse. Pension funds that might invest 5% of their assets in commodities can afford the hit. Uncle Phil will suffer more.