NEW YORK - The sight is so surprising that Americans are sharing photos of it, along with all those cute Halloween costumes, sweeping vistas and special meals: The gas station sign, with a price of $2-something a gallon.

"It's stunning what's happening here," says Tom Kloza, chief oil analyst at the Oil Price Information Service. "I'm a little bit shocked."

The national average price of gasoline has fallen 33 cents in October, landing Friday at $3.00, according to AAA. Kloza said the average will fall under $3 by early Saturday morning for the first time in four years.

When the national average crossed above $3 a gallon in December of 2010, drivers weren't sure they'd ever see $2.99 again. Global demand for oil and gasoline was rising as people in developing countries bought cars by the tens of millions and turmoil was brewing in the oil-rich Middle East.

Now demand isn't rising as fast as expected, drillers have learned to tap vast new sources of oil, particularly in the U.S., and crude continues to flow out of the Middle East.

Seasonal swings and other factors will likely send gas back over $3 sooner than drivers would like, but the U.S. is on track for the lowest annual average since 2010 - and the 2015 average is expected to be lower even still.

Trisha Pena of Hermitage, Tenn., recently paid $2.57 a gallon to fill up her Honda CRV. Like many around the country these days, she was so surprised and delighted by the price she took a photo and posted it on social media for her friends to see. "I can't remember the last time it cost under $30 to put 10 or 11 gallons in my tank," she said in an interview. "A month ago it was in the $3.50 range, and that's where it had been for a very long time."

Here are a few things to know about cheap gas:

- Crude prices came off the boil. Oil fell from $107 a barrel in June to near $81 because there's a lot of supply and weak demand. U.S. output has increased 70 percent since 2008, and supplies from Iraq and Canada have also increased. At the same time, demand is weaker than expected because of a sluggish global economy.

- In the past, a stronger economy in the U.S., the world's biggest consumer of oil and gasoline, typically meant rising fuel demand. No longer. Americans are driving more efficient vehicles and our driving habits are changing. Michael Sivak of the University of Michigan Transportation Research Institute calculates that the number of miles travelled per household and gallons of fuel consumed per household peaked in 2004.

- The drop from last year's average of $3.51 per gallon will save the typical U.S. household about $50 a month.

- The drop will save the U.S. economy $187 million a day, and also boost the profits of shippers, airlines, and any company that sends employees out on sales calls or for deliveries.

- It will take an extra 1.5 years to make purchasing a higher-priced, better-mileage Toyota Prius instead of a Toyota Corolla pay off.

- New York's average of $3.37 is the highest in the continental U.S. South Carolina and Tennessee are the lowest, with an average of $2.75.

- Politicians are either going to take the credit for lower gasoline prices or blame the other party for not helping them fall further. Don't listen. There are small things politicians can do over long time horizons, like implement fuel economy standards or ease drilling regulations, but the decline in prices is mainly due to market forces.

- Gasoline is cheaper than milk again. In September the national average price of milk was $3.73 per gallon. The annual average for milk is on track to be more expensive than the annual average for gasoline for the first time since 2011. The gap is even bigger for some bottled water lovers. A case of a dozen 1.5 liter bottles of Evian on Amazon.com costs $38.99, which makes for a price per gallon of $8.20.