[JayDee]

An interesting theory about aliens and why there aren't any flying around out there is because; a planet's sentient civilisation will only ever get one real chance to figure out a way to move past it's need to use finite resources as an energy source, and to use it's finite resources as a means to enter outer space.

If they can't then eventually as the finite resources are consumed, machines break down, industrial manufacture stops, and the civilisation is forced to revert back to a less technologically advanced time. Even if a new sentient civilisation were to rise on the planet, the planet wouldn't have enough resources for them to try again.

TL;DR You got one shot do not miss you're chance to go(colonise another planet), This opportunity comes once in a planet's lifetime, yo!

Also growing up there was this haunted house, that we always called "The Haunted Barn" that we'd visit often, and heard rumours that if you heard scraping on the wood or a banging on the bars inside the house then you'd have to leave. The entrance was boarded up so you could only ever really explore the outsides of the house. We'd go infrequently but we'd always hear the noises after sometime between half an hour to two hours - obviously being kids we'd always leave, but one time I went alone; forced one of the boards away and went inside. There were indeed scratches and claw marks at the wood, and the only metal bars I saw were those attatched to the staircase, I immediatly heard the noises again and cheesed it.
2Spooky4Mi