xkcd 2520

Symbols and what they mean

• d⁄dx — An undergrad is working very hard
• ∂⁄∂x — A grad student is working very hard
• ℏ — Oh wow, this is apparently a quantum thing
• Rₑ — Someone needs to do a lot of tedious numerical work; hopefully it not you
• (T_a⁴−T_b⁴) — You are at risk for skin burns
• Nᴀ — You’re probably about to make an incredibly dangerous arithmetic error
• ㎛ — Careful, that equipment is expensive.
• mK — Careful, that equipment is VERY expensive
• ㎚ — Don’t shine that in your eye
• ㋎ — DEFINITELY don’t shine that in your eye
• m㏜ — You’re about to get in an internet argument
• ㎎⁄㎏ — Go wash your hands
• ㎍⁄㎏ — Go get in the chemical shower
• π or τ — Whatever answer you get is going to be wrong by a factor of exactly two

https://xkcd.com/2520/ (xkcd alt-text) — “röntgen” and “rem” are 20th-century physics terms that mean “no trespassing.”