If you had a trillion dollars and a year to spend it for the good of the world and the advancement of science, what would you do? It's an unimaginably large sum, yet it's only around 1 percent of world GDP-and about the valuation of Google, Microsoft, or Amazon. It's a much smaller sum than the world found to bail out its banks in 2008 or deal with COVID-19.But what could you achieve with $1 trillion?You could solve the problem of the pandemic, for one, and eradicate malaria-and maybe cure all disease. You could end global poverty. You could settle on the moon and explore the solar system. You could build a massive particle collider to probe the nature of reality like never before. You could build quantum computers, develop artificial intelligence, or increase human life span. You could even create a new life form.Or, how about transitioning the world to clean energy? Or, preserving the rain forests, or saving all endangered species? Maybe you could refreeze the melting Arctic, launch a new sustainable agricultural revolution, and reverse climate change?How to Spend a Trillion Dollars is the ultimate thought experiment, but it is also a call to arms: These are all things we could do, if we put our minds to it-and our money.