Pipe dream...

Idiom: "pipe dream"

Definition:

an unrealistic hope; something that is unrealistic

Examples:

1) Becoming richer than Bill Gates is a pipe dream.

2) You want to become a professional football player?  That sounds like a pipe dream.

中文解釋：http://www.youzienglish.com/main/idiom_of_the_day/pipe_dream/cn/