Bill Nye was geeking out in his sunny apartment in uptown Manhattan.

“I can’t do anything without thinking about the science,” Mr. Nye said in early April, waving his arms around to make his point. “Look at this scene. We are on a Zoom call, which involves literally millions of transistors that somebody figured out how to manufacture.”

His voice got louder and more excited as he went on: “Then they got the T.T.L., the transistor-transistor logic, figured out so that we can make pictures and form words, and transmit them across the electric internet.”

Wearing his trademark short-sleeved white shirt, clear horn-rimmed glasses and a purple bow tie, Mr. Nye, 65, still looked like the former mechanical engineer who became America’s favorite science teacher during the 1990s, as the host of the PBS show “Bill Nye the Science Guy.”

Watched by millions of schoolchildren (and more than a few adults), Mr. Nye taught everything that there was to know in grade-school science and beyond, with wacky humor and zany demonstrations. In one episode about static electricity, he wears a heavy-metal rocker wig to show how negatively charged static makes hair stands up. (“Static electricity — it rocks!” he says.)

Now, 23 years since taping the last show, Mr. Nye has found a new audience and a lucrative gig as a celebrity pitchman.

It began about a year ago when he posted a 12-second video on TikTok, just as the first stay-at-home orders was taking effect across the country. Sitting at his messy kitchen table, he appears to be studying something under a microscope.

“I’m happy at home, just trying to save the world here,” he said, as he puts his eyeglasses on. “You know, it’s not as easy as it looks.” The clip generated more than 15 million views.

Many views likely came from millennials who grew up watching him distill complex scientific principles into short, fast-paced videos. “It was forced upon me in middle school when the science teacher didn’t want to teach a lesson, and they would roll in the TV on the cart,” said Ben Brainard, 25, a comedian in Orlando, Fla. “The theme song would get stuck into our heads for the rest of our lives.”

“I don’t think people ever really stopped caring about Bill Nye,” Mr. Brainard added. “They just kind of forgot about him in day-to-day life until now.”

In another popular TikTok video, Mr. Nye demonstrated why we need to wash our hands with soap. “Soap is an amazing substance,” he says. Using a loop of string floating on water to represent a coronavirus, he shows what happens when the loop makes contact with soap: “The molecule falls apart. That’s why you’ve got to wash your hands, wash your hands, wash your hands!”

Mr. Nye believes that his TikTok videos resonate with millennials in part because the videos are similar in format to the television show. “In ‘The Science Guy’ show, we had a hilarious old saying that no bit should exceed 49 seconds,” he said. “That was part of the charm — the rapid-fire presentation.”

His fans, he added, were also eager to embrace science at a time when science became politicized.

“We needed one of our role models from our youth to step up and say: These are the facts,” said Lauran Woolley, 26, a fifth-grade teacher in Canfield, Ohio, who follows Mr. Nye on TikTok. “Sometimes people don’t want to listen to science.”

For Mr. Brainard, it was also a matter of credibility. “It was during this time when nobody knew what was happening so they turned to the people they trusted, and for us, that was Bill Nye,” he said. “I will do anything Bill Nye tells me to do.”

That sense of trust has also prompted numerous brands to approach Mr. Nye to become their spokesman, especially brands that want to burnish their commitment to the sciences and climate change in particular.

“Bill is a world-renowned science educator and environmental advocate, and we are partnering with him to spread awareness,” said Dani Reiss, the president of Canada Goose, the outdoor apparel company, which hired Mr. Nye in April to be a sustainability adviser.

Mr. Nye said he has been approached by three to four companies a month during the pandemic but has declined most offers. “I prefer to work with brands that I use,” he said. “Is that madness?”

One of these companies was Bombay Sapphire gin, which may seem like an odd choice for someone associated with a children’s education show. But in 2014 Mr. Nye gave an interview to The Wall Street Journal, in which he revealed that his drink of choice was a Bombay Sapphire martini. “The Bombay Sappharians came after me after that,” Mr. Nye said. “I finally said I’ll do it this year.”

On April 9, he posted the first of three Bombay videos to his three million Instagram followers, a one-and-a-half minute clip in which he extols the “perfect pairing” of Bombay’s new gin-and-tonic in a can. “I don’t want to shock anyone, but I do enjoy a martini,” he says.

Fan reactions have been mostly positive, but also mixed. “This confirms that the Bill Nye target audience no longer needs to be carded at the bar,” one follower wrote. Others called him a “sellout” and questioned the health wisdom of promoting an alcoholic beverage.

Mr. Nye defends his work by stating something most of us take for granted. “When I pick up this glass of gin and tonic with the bubbles, I study it,” he said. “Science is empowering. It helps us appreciate the world around us.”