The CNN Democratic debate in Flint, Michigan, began with Hillary Clinton and Bernie Sanders both calling on Michigan Gov. Rick Snyder (R) to resign following the Flint water crisis.

Sanders spoke first, declaring that Snyder should resign due to his "dereliction of duty." "Amen to that," Clinton responded. "I agree, the governor should resign or be recalled. We should support the efforts of citizens trying to achieve that. But that is not enough. We have to focus on what must be done to help the people in Flint." She also called on the state to immediately send money to Flint for aid, and Sanders said it is "beyond belief" that children "in the year 2016 are being poisoned." Catherine Garcia

On Tuesday, President Obama nominated Jeffrey DeLaurentis as the first U.S. ambassador to Cuba in more than a half-century.

Since 2014, when the U.S. Embassy in Havana reopened, DeLaurentis has been the country's chief diplomat in Cuba, but he's able to get a promotion to ambassador now that the diplomatic freeze is over. "Jeff's leadership has been vital throughout the normalization of relations between the United States and Cuba, and the appointment of an ambassador is a common-sense step forward toward a more normal and productive relationship between our two countries," Obama said in a statement.

DeLaurentis must be confirmed by the Senate in a simple majority, but some senators, including Cuban-American Republicans Marco Rubio of Florida and Ted Cruz of Texas, have both said they will oppose any ambassador named by Obama. "A U.S. ambassador is not going to influence the Cuban government, which is a dictatorial and closed regime," Rubio said Tuesday. "This nomination should go nowhere." Catherine Garcia

The next time you have a kidney stone, don't go to the doctor or stay home in excruciating pain — head to your local amusement park.

After hearing from patients who said their kidney stones passed without pain after going for a ride on Big Thunder Mountain at Disney World, researchers at Michigan State University decided to conduct a test. Using a 3D printer, they created silicone models and put in kidney stones of various sizes, then took them for a ride. The scientists say even the largest stones were dislodged after two or three rides, and sitting in the back was more effective than being in the front.

Dr. Clayton Lau, a urologist at City of Hope in Duarte, California, told ABC Los Angeles the bumpiness of a roller coaster likely does not create enough turbulence to pass a stone, and the rush of adrenaline probably causes movement in the ureter, helping propel the stones. The researchers say their models show the stones did pass all the way after at least one ride, and suggest that people who are prone to getting kidney stones go for regular rides to keep them at bay. Lau doesn't see thrill ride therapy becoming the next big thing, saying, "I think that's the last thing you want to do when you're in pain is jump on a roller coaster," but ask any patient and even while hurting, they might choose Disneyland over the doctor's office. Catherine Garcia

Wells Fargo is opening an independent investigation into the company's retail banking practices, following the revelation that employees trying to meet sales targets opened as many as 2 million phony accounts for customers without their knowledge, the firm announced Tuesday.

During the investigation, Wells Fargo CEO John Stumpf will not receive a salary and will forfeit equity awards valued at around $41 million. Carrie Tolstedt, the former head of retail banking, has also left Wells Fargo, and will not receive a severance package. The firm says it has fired 5,300 people, mostly low-level employees, in connection with the fraudulent practices, and has paid a $190 million fine. It also will eliminate sales goals in retail banking on Jan. 1, 2017. Catherine Garcia

On Tuesday, Syrian government troops and allies attempting to take over the rebel-held side of Aleppo stepped up their ground attacks in the Old City.

The move comes after the collapse of a ceasefire, backed by the United States, after just one week, and the U.S. says it's proof Syrian President Bashar al-Assad will do anything to defeat the opposition, ignoring the peace process. Over the past week, hundreds of people in Aleppo have been killed in bombings, including 12 members of two families on Tuesday. It's thought that more than 250,000 people live in the besieged part of Aleppo, with only 30 doctors remaining to take care of the hundreds of people wounded daily.

Before the civil war began nearly six years ago, Aleppo was the biggest city in Syria. The army has control of the western zone, and soldiers and militia fighters supporting Assad told Reuters they are starting to move in armored vehicles and tanks for more attacks against rebels. Catherine Garcia

The first baby has been born using technology that allows for the combination of DNA from three different people, CBS News reports. The procedure, which is illegal in the United States, was performed by a team led by Dr. John Zhang of the New Hope Fertility Center in New York at a facility in Mexico; the baby boy is now six months old.

The mother carried a genetic mutation for Leigh syndrome, a neurological disorder that is often fatal within a few years. Two of the mother's children had died from the syndrome, and she'd had four miscarriages. The new baby has so far been healthy and showed no signs of the disease thanks to the "revolutionary" technique, which "involved removing some of the mother's DNA from an egg, and leaving the disease-causing DNA behind," The Associated Press reports. "The healthy DNA was slipped into a donor's egg, which was then fertilized. As a result, the baby inherited DNA from both parents and the egg donor."

“This is the very first time at least in human reproduction that the offspring are produced with three parties — one sperm and different parts of two eggs ... So this is very revolutionary,” Zhang told CBS News. And while the procedure might not be allowed in the United States, Zhang insisted, "To save lives is the ethical thing to do." Jeva Lange

The mission to make humans "a multiplanetary species" just got a little bit closer to becoming a reality. Elon Musk's SpaceX program published a video Tuesday describing how the proposed Interplanetary Transport System would actually work — and it involves 28,730,000 pounds of thrust for lift-off, the support of a refueling pod, and winged solar panels to provide additional power to the ITS:

Musk is delivering a presentation on colonizing and building a city on Mars at the International Astronautical Conference in Guadalajara, Mexico (you can follow continual updates at The Verge, here). And for those dreaming of future travels to a terraformed Red Planet, you're in luck — the planet makes an unnaturally blue-and-green appearance at the end of the video.

Musk also said Tuesday that he believes in the future the cost of travel to Mars will be as inexpensive as buying a house. SpaceX plans to send its first ship of humans to the planet as early as 2024. Jeva Lange

Mother Earth isn't the only one suffering from rising levels of air pollution. A new report by the World Health Organization out Tuesday revealed that in 2014, 92 percent of the world's population was living with levels of air pollution that exceeded what WHO considers to be safe.

In particular, WHO is concerned about higher concentrations of pollution particles that are smaller than 2.5 micrometers, as these particles are tiny enough to "be inhaled, travel into the lungs, and enter the bloodstream," The Washington Post reported. "People think of air pollution as a respiratory disease," said Carlos Dora, head of WHO's air pollution team. "And in fact, it's heart disease, strokes and cardiovascular. Because there's very small particles that go into the blood. ... The damage air pollution does to the vessels is similar to the damage that cholesterol or high blood pressure do."

In 2012, an estimated 7.3 million people died from air pollution, produced both inside the home and outdoors. The problem is particularly acute in "low- and middle-income countries," which WHO reported accounted for 88 percent of the 3 million premature deaths caused by outdoor air pollution in 2012.

Still, wealthier countries certainly aren't in the clear. In Europe, cities including Paris and London failed to meet air quality standards, and in America, both Los Angeles and Manhattan fell short. Becca Stanek