list sorting question

[DPaul]

I was checking up on sorting 2D lists, and found this (works fine):

#sort on first element
def takeFirst(elem):
    return elem[0]
# list
lst = [(2, 2), (3, 4), (4, 1), (1, 3)]
# sort 
lst.sort(key=takeFirst)
# print list
print('Sorted list:', lst)

At first i thought that takeFirst, takeSecond, were some kind of "reserved" words,
but when i replaced takeFirst with "chicken", it also worked !

Question: by what mechanism does takeFirst() know what "elem" is?

thx,
Paul