$NVDA hit all-time highs today. You could have seen it coming months ago. Amazon, Alphabet, Microsoft and Meta plan to spend about $725B on capex in 2026. Capex is the money a company spends building things it will own and use for years, as opposed to its day-to-day costs like salaries and marketing. For most companies that's factories, trucks, office buildings. For these four right now, almost all of it is going into one thing: AI data centers. An AI data center is a warehouse-sized building packed with racks of servers, and each server is built around a stack of GPUs, the chips that train and run AI models. Microsoft needs them to run Copilot and sell compute to OpenAI. Alphabet needs them for Gemini. Meta needs them for Muse and ad targeting. Amazon needs them to rent out through AWS to Anthropic and everyone else who wants AI compute without building their own data center. Nvidia doesn't collect $725B. Some of it pays for land, power, cooling and networking. But the biggest line item is the GPUs, and Nvidia sells roughly 80% of the AI chips going into these buildings. That's why its data-center business is running at about $355B a year, up 100% from last year, close to half the entire capex line. So capex doesn't cause Nvidia's revenue one for one. The correlation, though, is about as tight as markets get: since 2022 the two lines have moved together every year, and Nvidia's slice of the pie has grown from 10% to roughly half. When one of these four raises its capex guide, the next big $NVDA earnings report is already being written.