The hottest new trade is the most boring asset class: money market mutual funds. Once regarded as a no-yield option investors would only use to park cash in a crisis, they now yield more than 4.5%. In the first quarter alone, investors flocked to them to the tune of half a trillion dollars. But how do they work? What do they invest in? And why do investors use them instead of similar-yielding, cheaper Treasury exchange-traded funds?

We answer all these questions and more on the latest episode of Trillions, which features Nafis Smith, principal and head of Vanguard’s taxable money markets, as well as Bloomberg Senior Editor Mike Regan and reporter Katie Greifeld.