Besides quantum mechanics and Keynesian economics, what are the things that make Post readers go hmmm?

Pigeons, ambulances and hamburgers!

After answering some burning questions about New York City last Sunday (do pedestrian walk signals actually work? Why does a “regular” coffee mean with milk and sugar?) we asked readers for the little mysteries that puzzle them. Here, some of the questions they submitted:

Why does The Bronx have a ‘the’ in front of its name? — John, Petersburg, NY

“That’s the most asked question,” admitted Bronx borough historian Lloyd Ultan, a history professor at Fairleigh Dickinson University. Sweden-born merchant sea captain Jonas Bronck purchased land opposite today’s Harlem around 1639, and when he died in 1642, the only thing that bore his name was the Bronck River, which cut through his farmland.

The ever-efficient British later shortened the river’s name, dropping the ‘ck’ and tacking on an ‘x.’

When today’s Bronx was annexed to NYC in 1874 and 1895, the city needed a name for the darn thing, so planners looked at an old map, and saw in the middle of the amorphous territory The Bronx River. So they named the land The Bronx, after the river, a name that took hold officially in 1898, Ultan said.

Interestingly, Bronx County does not contain “The.” Originally, The Bronx was considered part of New York County, but Bronx residents pushed for a county all their own — for convenience, mostly. After all, traveling to the southern tip of Manhattan for a visit to the county court was not as simple as hopping on an express train . . . Anyway, in 1914, they got their own county name, “but [designators] forgot that there was a “The” in front of the name,” Ultan noted. Hence, Bronx County.

Is it illegal to feed pigeons? — LPaige

Depends on whom you ask. The Department of Health says it’s perfectly legal, except in certain parks where signs forbid the practice. But the Department of Sanitation says that throwing food on the ground is littering, and violators may receive $100 fines.

Citing the damage done by pigeon poop, former City Councilman Simcha Felder proposed a $1,000 fine for feeding them in 2007 but the plan never took flight. Felder, who will begin his first term as a state senator next month, told The Post he has no plans yet to introduce a similar measure.

What are those orange circular lights that are never on but on the rear of all taxi cabs? — Andrew, Tinton Falls, NJ

The orange is a Code Red. “The idea is that if a driver is concerned for his or her safety, the light can be activated out of sight of the passenger, so that police officers — who are trained to know what these lights represent — may be alerted,” said Allan Fromberg, a spokesman with the Taxi and Limousine Commission. The amber-colored lights — located on the front center of the vehicle, and on top of the rear bumper, to the left of the license plate — flash between 60 and 120 times per minute. The activator is typically installed within easy reach of the driver.

What are those casino-chip-looking discs embedded in the asphalt? — Aubrey

The discs are identification tags placed on a roadway’s surface after a contractor has completed work there, according to the Department of Transportation. Each tag includes the contractor’s name and the date of the restoration, and the markers allow the agency to better monitor conditions after repairs and notify a contractor if adjustments are needed. The colors range from cherry red, for Verizon, regal blue for cable TV, and purple for the MTA.

Why do I always see ambulances on the street just standing there with their engines running? — Toyota

“They are not just ‘standing there,’ that’s where they are posted waiting to respond to the more than 1.4 million medical emergency calls FDNY EMS receives each year,” said Fire Department spokesman Frank Dwyer, who credited the policy for cutting response times to the most serious medical emergencies — cardiac arrest, respiratory arrest — to 6 minutes and 31 seconds, in 2011.

The engines are often kept running due to the medications kept on board. “The trucks run to keep equipment working . . . and keep items carried at a safe temperature like medication and IV fluids and to keep the cab at a comfortable temperature for the crew,” a retired FDNY EMT said.

What are all these MTA buses that are sitting around my area for years rotting away? — Toyota

When buses go to die, they go to The Bronx.

“The Eastchester Depot is the location where we scrap retired buses for parts that we restock for use when a part is needed on a bus in the active fleet,” said MTA spokesman Kevin Ortiz. The remaining unserviceable parts are scrapped — sold as a commodity to be shredded and then used to form new metal products. Typically, there are around 150 to 200 buses at any given time awaiting scrap processing. So far this year, the MTA scrapped 559 buses, generating $1.4 million in revenue, Ortiz said.

Why don’t McDonald’s hamburgers come with mustard in NYC? — Steve, Manhattan

Unlike most of the rest of the country, mustard is indeed not standard on an NYC McDonald’s burger.

A company rep chalked up the absence to “customer preference,” but would not say how or why the company arrived at that conclusion. They simply believe New Yorkers just don’t like mustard on meat. You can ask for it, though, or drive to Jersey.