How does "Let's Encrypt" provides authenticity?

http://ift.tt/1k1WMFo's_Encrypt

http://ift.tt/1Ddkjiy

AFAIK with "Let's_Encrypt", we could create HTTPS websites with only one command.

Question: But what provides authenticity, if anybody could create a valid HTTPS cert to any domain? How does Let's_Encrypt ensures that I am the domain owner, and not an attacker that want to MITM the domain?