What does the expression mylist[0] return?

The expression mylist[0] returns the first element of the list because Python uses zero-based indexing. This means that the index of the first element is 0, the second element is 1, and so on. Therefore, when you access an element of a list using square brackets and an index, such as mylist[0], you are effectively retrieving the element located at that position in the list.

In contrast, attempting to reference the last element directly would require specific knowledge of the list's length or using a negative index, while the second element would be accessed with an index of 1, and a slice of the list involves using the colon operator to specify a range of elements rather than a single indexed element.