Filling up at the gas station has been a much more pleasant experience for Americans since last fall. Regular gas is now less than $2 a gallon in many states, down from around $3.30 just a year ago.

But how long will that last? It's just one of many questions stemming from the extraordinary drop in crude oil prices — a development that has boosted consumer confidence, hurt once-booming energy states and presented new opportunities — and challenges — for the U.S. and global economy.

Q: Why did oil prices fall so much, so fast?

A: A confluence of factors has contributed to the more than 50 percent slide in oil prices since September. The biggest is the steady rise in world petroleum supplies, mainly because of the shale-oil revolution in the U.S. Thanks to hydraulic fracturing, or fracking, and other drilling techniques, the U.S. has accounted for more than 80 percent of global crude production growth in the last five years. More recently, an increase in oil output in Iraq and Libya has further boosted capacity.

At the same time, there are signs of softening demand. Economies in Europe and Japan have been stagnant, and the Chinese economy, the biggest driver of global oil demand, is slowing down. The strong dollar also has helped pushed down oil prices.

Q: How long will it last?

A: Low prices at the pump may be short-lived because the cost of crude is likely to start rebounding in the second half of this year. That's based on predictions of future supply and demand, including that low prices will stimulate greater use and therefore lead to an increase in demand. Oil futures lately have been trading at about $45 per barrel, compared with an average $100 in the first half of last year. By this time next year, Moody's Analytics estimates that oil will bounce back up to $80 a barrel.

But there are other factors that could come into play. The fall in