Today is Bill Gates’ 61st birthday, and in true sharing fashion Mark Zuckerberg took to Facebook to wish Gates a happy birthday and thank him for his friendship.

This comes a few days after Gates, cofounder and former chief executive of Microsoft, said that he and the Facebook cofounder and chief executive are “good friends.” Gates gushed about Zuck in a conversation on October 20 with Robert Perkins, a content and media strategist from the office of strategic communications at California Institute of Technology (Caltech) in Pasadena, California, after a question-and-answer session with the dean of undergraduate students at the school. Perkins told VentureBeat that he duly transcribed the conversation before publishing it on Caltech’s website.

The bond between Zuckerberg and Gates is not a hard thing to imagine. In a 2013 interview, Zuckerberg said that Gates was his “hero” when he was growing up and that he saw Gates as “one of the greatest visionaries that our industry has ever had,” even thought Microsoft itself had “lost some of the focus” it had had earlier.

Plus, in 2007, Microsoft invested $240 million in Facebook. Over the years the two companies have integrated their products in various ways — Bing Maps came to Facebook, for instance, and more recently Facebook has released native Facebook, Instagram, and Messenger apps for Windows 10.

On top of that, Gates and Zuckerberg both have an interest in philanthropy, among other things. Gates — who with his wife started the Bill and Melinda Gates Foundation in 2000 — was on hand at the Chan Zuckerberg Initiative event last month where the organization announced that it would spend $3 billion to cure diseases.

Here’s exactly what Gates said about his connection with Zuckerberg on October 20:

Well, there is an irony that I went back to Harvard and gave a speech and Mark Zuckerberg was in the audience [laughs]. Now, I don’t claim it had an influence on him, and he and I are good friends, but it was kind of a funny thing.

See the rest of Perkins’ conversation with Gates here.