Noob question about lists

[adifrank]

Hi. Noob question (sorry)...

I'm learning Python myself (reading "Head First Python").
Having trouble wrapping my head around why in the below code the integer '5' is removed from 'list_a', even through the pop() method was applied only to 'list_b'.

list_a = [1, 2, 3, 4, 5]
list_b = list_a
list_b.pop()
print("list_a is", list_a)
print("list_b is",list_b)

I understand that the line stating "list_b = list_a" makes the two variables have the same value.
But after that, I expected that 'list_a' and 'list_b' are each their own individual object and that changes made to one wouldn't affect the other.
In other words, I expected the value of 'list_a' to remain [1, 2, 3, 4, 5]

Can someone please help explain this to me?
Thanks