Now, research suggests that the soft, organic scaffolds in which such crystals form guide crystallization by soaking up the calcium like an "ion sponge," according to new work in Nature Materials.

Whether it’s the Fitbit Ionic, Versa 2, Charge 3, Inspire, or Ace 2, here’s an easy way to decide which Fitbit model you should buy. It all comes down to what feature matters the most.