The stage lights stayed down as the introductory video ended, and a lone figure walked onto stage in the darkness. When the lights came up, there was Bill Gates, and the crowd let up a big cheer at the surprise. But this was not the Gates they were waiting to see on this night.

“I get to introduce Melinda for the last stop on her book tour,” the Microsoft co-founder said, adding that he was a bit nervous about the prospect of introducing his wife, and deciding what gift to give her as she walked on stage. so he decided to get some advice from someone with experience in such matters.

Up came a video of former President Barack Obama offering his advice based on his experience introducing his own wife, suggesting everything from flowers to an ice sculpture.

Never in a million years would I have guessed @BillGates was secretly scheming with @BarackObama to surprise me on the last night of my book tour. An unforgettable end to an unforgettable experience. #MomentofLift pic.twitter.com/2Eu1KiplP1 — Melinda Gates (@melindagates) May 10, 2019

But ultimately, Bill Gates said, he decided on something else.

“My gift to Melinda is simply to tell her that the moment we met was my moment of lift,” he said, holding back tears on stage.

On a warm spring night when it might have been tempting to stay outside, a crowd of more than 2,500 people filled McCaw Hall in Seattle on Thursday night, to hear Melinda Gates share insights from her years of work in technology and philanthropy — discussing the central thesis of her book, “When we lift up women, we lift up all of humanity.” It’s a thesis informed not just by her own career but by her on-the-ground experience around the world, visiting and living with women in cultures with traditions of suppressing them.

Those cultures include the United States. In the book and on stage, Gates discussed one of the central challenges women around the world face: unpaid labor.

“If you look at the unpaid labor that we do in our homes, some of it’s the caring for loved ones we want to do but [much] of it is chores,” Gates said during the on-stage conversation with Jessi Hempel, a longtime journalist who now works for LinkedIn. “It’s doing the dishes and the laundry, and shopping, and helping kids with homework. That unpaid labor across the world, if you average it out, is seven years of a woman’s life, and I know a lot of women that could do a lot with seven years of their life.”

In her book, Gates notes that a woman could earn a PhD in the time she spends on unpaid labor. It’s one example of how inequity between genders leads to fewer opportunities for women.

Gates has seen this dynamic play out around the world through her work with the Bill and Melinda Gates Foundation. One example she highlights in the book is the story of Grace, a young girl in Tanzania. Grace is at risk of failing the exams needed to advance her education because of the long hours she’s expected to work in the home. Her twin brother passes his exams.

Packed house at McCaw Hall in Seattle tonight for hometown finale of ⁦@melindagates⁩ #MomentofLift book tour. ⁦@mnickelsburg⁩ and I interviewed Melinda earlier today for a GeekWire podcast, so stay tuned! pic.twitter.com/1KZcqnWdRO — toddbishop (@toddbishop) May 10, 2019

“There are millions of girls like Grace, and their extra share of unpaid work could make the difference between a bright and flourishing life and a life of cooking and cleaning and never having time to learn and grow,” Gates writes in “Lift.”

Labor isn’t quite so stratified in the U.S., but women still end up doing about 90 minutes of extra unpaid work a day, according to Gates. She has become a leading advocate for paid family and medical leave in the U.S. as one way to balance the scales.

During the event Friday, Gates said she’s become “very passionate” about enacting a federal leave law because “we are the only, the only industrialized nation that does not have paid family medical leave.”

That’s one benchmark she uses to measure progress toward gender equality in the U.S. Another is the business community.

“When these four industries eventually tip is when I think we’ll get there as women,” she said. “I look at finance. I look at the media. Who tells our stories in our society? I look at politics and who makes our laws. And then I move to tech because tech is what is creating the future.”