Tesla CEO Elon Musk celebrated his son’s birthday over the weekend with a heartfelt post on his social media platform, X. The 52-year-old, who acquired X, formerly Twitter, for $44 billion in 2022, shared an endearing snapshot of his son, X Æ A-Xii, to commemorate his fourth birthday.

In the post, Musk captioned the image: “Lil X turns 4 today on May 4th.” The photo captured X Æ A-Xii flashing a smile while sporting a Tesla Cybertruck shirt, a nod to his father’s innovative ventures.

Musk and his former partner, Canadian musician Grimes, welcomed their baby boy into the world in May 2020. The couple’s unconventional choice of name for their son sparked curiosity. Grimes explained that the “X” represents “the unknown variable,” while “Æ” symbolizes AI, standing for “artificial intelligence” and also translating to “love” in certain languages like Japanese. Additionally, “A-12” pays homage to the SR-71 Blackbird, a favorite aircraft of Musk and Grimes, known for its speed and agility in combat situations.

Moreover, Grimes elaborated that the “A” in their son’s name signifies “Archangel,” inspired by a song titled “Archangel” by Burial, a personal favorite of hers.

Musk’s affectionate gesture and the unique backstory behind his son’s name have garnered attention, reflecting the couple’s distinct approach to parenthood and life.