Both "What are Little Girls are Made Of?" and "Dagger of the Mind" involve
a famous scientist who turns bad,
who traps Kirk and a woman companion with a past love interest deep underground
a rotating, consciousness-stealing, evil technology
the same sound effects are used to depict the spinning device
beaming an unauthorized, mind-washed person aboard the Enterprise.
Spock discerns what the problem is by picking up clues from mind-washed individual
the ultimate death and destruction of said scientist.
Which episode do you like better? Why?