Mario Alioto says he knew instantly on May 9, 1999, that the San Francisco Giants had a good idea on their hands.

Alioto was the team’s vice president of sales and marketing, and the Giants were playing the Milwaukee Brewers at 3Com Park (better known as Candlestick Park, the team’s home since 1960). First pitch wasn’t until around 1 p.m., but fans had started showing up hours before the gates even opened.

“I remember we got there about 7 in the morning,” said Robert Villegas, who’s been a Giants fans for nearly four decades. “It was a fun day because there was the Willie Mays bobblehead.”

The first 25,000 fans in the doors received a free Willie Mays bobblehead. While this might seem like a run-of-the-mill promotion today, back in 1999 it had never been done before.

“May 9, 1999, is really the day that transformed bobbleheads,” said Phil Sklar, co-founder and CEO of the National Bobblehead Hall of Fame and Museum.

Bobbleheads first gained popularity in the 1960s when they became a staple at stadiums and corner stores around the country.

“You’d go to the souvenir stand, and there was a bobblehead, and if you bought one or your parents bought one, that was a big deal,” said Alioto, now the team’s executive vice president of business operations, recalling his days as a young Giants fan.

The dolls faded from popularity over the next three decades, but when it came time for the Giants to celebrate their 40th and final season at Candlestick Park, Alioto and Valerie McGuire, the team’s director of special events, thought it would be fun to give away a bobblehead as a way to hearken back to the stadium’s early days.

But actually getting the bobbleheads made proved challenging. When the team reached out to one of its suppliers, they had no idea what a bobblehead even was. The first prototype they sent was tall and skinny with a tiny head.

“When we first saw that, we laughed,” Alioto said. “We said that’s not the idea. It’s got to be almost like Bob’s Big Boy. It’s got to be chunky and a heavy doll.”

It took a few rounds of back-and-forth before the team finally saw a design they were happy with.

While the actual production process might have been difficult, it wasn’t hard to choose which player would be the first bobblehead.

“There was no doubt if we were going to do the first one, it would be Willie Mays,” Alioto said.

Almost immediately after the Giants’ inaugural giveaway, other teams jumped on the bobblehead bandwagon. Within a few years, clubs in every major professional sport in the United States were giving them away.

According to Sklar, the limited number of bobbleheads handed out before a game and the popularity of the online auction site eBay helped turn bobbleheads into instant collectibles. In 2015, a New York Yankees bobblehead from the early 1960s sold for nearly $60,000 at auction.

Bobblehead technology has advanced significantly over the last 20 years. Much like video games, the likeness between the actual players and their bobbleheads has improved dramatically. (Alioto said that to this day, Mays insists the 1999 bobblehead looks nothing like him.)

The Giants have done bobbleheads of Tony Bennett and Frank Sinatra with sound chips, a Barry Bonds bobblehead with a home run counter, and another Mays bobblehead celebrating his famous over-the-shoulder catch that included a ball on a string about to drop into his glove.

“I always wanted to do one with Juan Marichal if we could do the high leg kick, or with Rod Beck where he would swing his right arm, so instead of the head bobbling maybe the arm bobbles,” Alioto said.

It’s not just the sports world that has caught bobblehead fever over the last 20 years. Alioto remembers going on a family vacation to the Vatican and seeing someone selling pope bobbleheads.

“I thought, ‘Boy, now I know this idea really worked,’ ” he said.

And the phenomenon shows no sign of slowing down. Sklar said there have been more than 7,000 bobblehead giveaways in the United States since 1999, and he sees kids as young as 4 and 5 years old toting collections with upward of 100 dolls.

“I don’t think it’s anywhere near fading off,” Sklar said. “I liken bobbleheads more to Barbie as opposed to Beanie Babies.”

Last month, the Giants gave away a replica of the 1999 Mays bobblehead in celebration of its 20th anniversary, one of six bobblehead giveaways the team has planned for this year.

“We’re all looking for the next bobblehead, and we haven’t found it yet,” Alioto said. “So while they’re still working, we’re still producing bobbleheads.”