正外部性

正 = 一 + 止 : Mnemonic symbol: a stick-figure with straight lines. James II of England (zh) really has to use the engine's bathroom ((e)ng4), but there's a stick-figure (正) blocking the stalls with a barrier (止). The stick-figure measures with a flute (一) whether James is tall enough to go. James stands very straight (正) to make himself as tall as possible.

外 = 夕 + 卜 : There's a hole in the wall of the airplane's bathroom (ai4), and from outside (外), a bat (夕) wants to enter. Willy Walrus (w) is very afraid of bats, so he does his best to hinder the bat from entering with a voodoo staff (卜).

部 = 咅 + 阝 : Bruno Bear (bu) has been harassed by the Imperator (阝) in the space station's bathroom (Ø4), so Bruno sets up a trap. He creates two fake boobs (乳部) out of poo-poo (咅) and as the Imperator grabs his boobs, the Imperator realizes his mistake and exclaims "bah!" (部).

性 = 忄 + 生 : Marie Curie had a strong character since she was little. Even in her cradle in the engine's bathroom she was only playing with a flail.