Henna (also called mehndi, anella, or lalle) is a temporary “tattoo” and hair dye made from the henna plant (Lawsonia inermis). People in many cultures use it to create elaborate designs on the skin.

I first met Ruby Bansal at school when we were nine years old. Through high school chemistry, college life, and budding science careers in different parts of the U.S., we kept in close contact. In ...