Fermi's Paradox

The story goes like this: Enrico Fermi talked about the possibility of extra-terrestrial intelligence with his contemporary physicists. they were fascinated that our galaxy holds 100 billion stars, that life evolved quickly and progressively on earth, and that an intelligent species could colonize the galaxy in just a few million years. They reasoned that extra-terrestrial intelligence should be common by now. Fermi listened patiently, then asked, "So, where is everybody?". If extra terrestrial intelligence is common, why have the bright aliens not landed in our planet yet? This conundrum is became known as the Fermi's paradox.