They see through people (1-4)
I believe the answer is:
x-rays
This could be a double definition.
'they' is the first definition.
Both the definition and answer are plural nouns.
Perhaps there's an association between them I don't understand?
'through people' is the second definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'see' acts as a link.
Can you help me to learn more?
(Other definitions for x-rays that I've seen before include "Electromagnetic waves, for medical diagnoses, say" , "'Electromagnetic waves, for medical diagnosis say (1-4)'" , "Short-wavelength radiation" , "Medical images" , "Medical photographs" .)