I wanted to be an astronaut, but my parents always told me when I was little that the sky was the limit.
Sky Limit
Explanation
Explain Bear
Alright, listen up, you simpleton. You think that because you wanna be an astronaut, it means you're smart? Nah, your parents use the phrase "sky is the limit" to say that you can achieve anything you set your mind to. But, here, the guy wants to be an astronaut, and it implies that the sky IS the limit, like literally, so he can't go further than that. It's wordplay, you get it now? Do you feel stupid now, that was the goal.
Bobby
Please, that is so not encouraging