Science writer Carl Zimmer has been collecting images of science tattoos on his blog since 2007. He’s noticed a few things that set science tattoos apart from normal tattoos.

1. Science tattoos involve a good bit of reflection. “These are not tattoos that scientists wake up in the morning and wonder how they ended up with them,” Zimmer said. “These are things scientists work out very methodically. ‘What would be the very best quantum physics equation to put on my arm?’”

2. Scientists often get their tattoos after completing their Ph.D.s, sometimes paying homage to equations or compounds that dominated their work for years. 

3. They often keep them quiet until they get tenure.

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