There are the three neighborhoods that this graph lives in:
Let's look at the left neighborhood:
* Remember your 2 sentences!
Since he's a function (and must pass the vertical line test), he can't do both.
What did we say about crossing the x-axis for this guy? (Look back if you need to.)
Yep -- he's not allowed to cross the x-axis! But, look at that bottom guy:
But, he's not allowed to -- so, he's got to live upstairs!
We can use the same reasoning for the right neighborhood: