ELI5 Silica Gel Desiccants - can they be reused?

What exactly are desiccants, and why are they commonly used in packaging? How does silica gel effectively trap moisture in humid environments? Can you explain the nanoporous structure of silica gel that makes it so efficient? Why do some silica gel packets change color when they reach capacity? Is silica gel really safe to use? Can you reuse them?

... we explain like I'm five.

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: Em_Adespoton, DiamondIceNS, intchd, Gnonthgol, Bosstiality, LCIronmanX, Mr_Automatic, stugautz.

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