"For my #flashbackfriday I want to send birthday ? to my favorite birthday girl @therealselmablair," Gellar captioned the slideshow, which started with their iconic Cruel Intentions kiss. "So Selma.....kissing burns 6.4 calories a minute. Want to workout?!? ?."
Gellar hasn't just remained friends with Blair since Cruel Intentions was released in 1999, she's also still close with Reese Witherspoon.
"Reese's son and my son are best friends," Gellar told ET in 2015, adding that, sometimes they even carpool together.
"We all see each other, so you don't realize you're getting older," the 40-year-old actress said. "Then all of the sudden you're faced with the revival of your movie, with all these young kids in it, and were like, 'Oh, we're old.' That's when it hits you."