Slack is accusing Microsoft of ripping off its ads. In a tweet posted by Slack today, the chat company makes a clear comparison between its own Frontiers video from earlier this year and Microsoft’s latest Teams promotional video from earlier this week. Both feature virtual colorful balls that are supposed to represent communication. Slack even hits Microsoft with the “ok boomer” meme to add insult to injury, mocking Microsoft for supposedly being old and out of touch.

Microsoft revealed its latest promotional video earlier this week, alongside an announcement that its Teams app is now being actively used by 20 million people daily. That’s a massive jump from the 13 million daily users the company reported back in July, and it’s almost double Slack’s user count of 12 million that it reported last month.

Slack has consistently questioned Microsoft’s growth, pointing toward engagement and the company’s focus on people. Some have wondered if Microsoft’s numbers are slightly inflated. But earlier this week, Microsoft shared some insight on how it counts “active users.” Jared Spataro, VP of Office 365, said:

“We define DAU as the maximum daily users performing an intentional action in the last 28-day period across the desktop client, mobile client and web client. Examples of an intentional action includes starting a chat, placing a call, sharing a file, editing a document within teams, participating in a meeting, etc. We strip out passive actions like auto boot, minimizing a screen, or closing the app. We also de-dupe all actions across a single user ID.”

Either way, both companies are caught in a war over which one will win the workplace. Microsoft launched its Slack competitor three years ago, and it was enough to force Slack to put out a full-page ad in The New York Times welcoming Microsoft to the party. It was clear back then that Slack was worried about the competition, and that has only intensified in recent months.

Microsoft claimed earlier this month in an interview with The Verge that Slack doesn’t have the “breadth and depth that’s really required to reinvent what it looks like to work together.” Slack has been popular among businesses that are looking to find more modern ways of working, but Microsoft’s aggressive push with Teams means a lot of businesses will opt for Teams simply because it’s bundled as part of Office 365. That’s Microsoft’s big advantage, and the company argues it’s expensive and complex to bring together a combination of Zoom, Slack, Google, and Dropbox instead of subscribing to Office 365.

Slack claims it’s not worried about the reach of Office 365, and it’s focused on the amount of time people spend using its apps and the love people have for its product. Slack’s tweet is curiously timed, though. Slack’s stock price plunged 11 percent earlier this week after Microsoft revealed its latest Teams numbers.

Update 1:00PM ET November 21st: The article has been updated with recent details of how Microsoft counts daily active users on Microsoft Teams.