Fans can't wrap their heads around it

The actor (52) shares his teenage twins with his husband David Burtka (50), whom he married back in 2014 after debuting their relationship in 2007.

In 2010, the pair became parents to twins Gideon and Harper through a surrogate who carried and gave birth to them.

Fans can't seem to get over the fact that the twins are already due to attend high school. One user joked in the comments: "Hey, I’m pretty sure they were just toddlers a few weeks ago, so there’s no way."

Another user wrote: "I’m sorry, what? I remember the coordinated Halloween when they were babies."

Harris shared a photo of the twins looking up over their laptop screens, as he captioned it: "The kids are entering high school this year (how wait what??), and they are at last allowed to rid themselves of their Chromebook anchors and ascend to the world of the MacBook. Huzzah!"

As fans are noticing the twins growing up on Harris' social media, they always seem to point out that one of the twins has quite the resemblance to Harris.

The kids are non-identical, as two separate eggs were used, however, fans can't get over how much the twins look alike and their resemblance to their dads.

One user wrote: "No doubt whatsoever whose kid is whose biologically lol," however, someone else disagreed, writing: "Oh I disagree I think she’s Neil’s & he’s David’s lol."

Another user wrote: "Gideon looks more like David, his sister looks more like NPH."

However, others just can't get their heads wrapped around it: "How is it that they honestly look like both of you?! They're gorgeous," and "These two are so similar to both, it’s impossible to tell who’s biologically who’s. That’s so cool."

Harris and Burtka both donated sperm in the hopes that either would fertilise the egg, and much to their surprise, the twins were conceived.

Back in 2014, Harris told ABC News: "We inserted one of my sperm and one of David's sperm into two eggs with the hope that they would both take, just because we both wanted to be dads biologically. Both took, miraculously."

He's also said that he has no interest in finding out which twin is biologically his.

The actor explained: "I have no interest in [finding out]. We are their parents and I love them implicitly."