Space visitors

"Greetings, Earthlings. We come from the star you call Alpha Lyrae, or Vega."

"Damn Vegans."

"Why do they always have to announce themselves?"

Greenpeace at Nazca

Occam

"I photographed this UFO in the countryside last night. It was like a fireball blazing in the sky."

"I see."

"What do you think? Is it a meteorite, or something else?"

"Well, Occam's Razor tells us we should always prefer the simplest explanation. So, which one is likelier? That a lump of space rock has been floating around for millions of years, avoiding all that time to hit other heavenly bodies, just to fall to earth at the right time and place for you to take a picture of it?"

"Hum... if you put it that way..."

Demarcation

"Yes, we can agree that it's an object. However, it doesn't look to me like it's flying. And it's pretty much identified, too. I'd appreciate it if you stopped wasting my time."

Unidentified

"It was a point of light on the Western horizon, short after sundown. It was hovering there for a while and then went out of sight behind the trees."

"Well, the first thing is to keep an open mind and not rule out any hypothesis, no matter how outlandish it may seem."

"Someone told me it could be Venus."

"Oh, yeah! Right! Pffft."

Photographic evidence

"The usual skeptics must know that NASA itself examined this photo and certified that there's no digital manipulation."

Identification

Stealth

"If aliens don't have an impossibly advanced camouflage technology, then how to explain the fact that this UFO disguised as a cloud is perfectly indistinguishable from a real cloud?"