Far from home, competing against the best in the world, with the eyes of billions of people on them, Olympic athletes are under the kind of pressure that few of us can imagine.
But they're only human and they have needs just like everyone else.
Some of the world's biggest sports stars from the past and the present on what they just can't do without when they're at the Olympics.
Justin Gatlin, U.S. sprinter and 2012 100m bronze medal winner:
"Communication with my son. I have a two-year-old boy and obviously he doesn't know what's going on now, the magnitude of it, but he knows that Daddy runs... to hear him talk to me and tell me he loves me before I go out and compete, that gives me the mojo to go out there and do the best I can do."
Michael Phelps, U.S. swimmer and winner of the most medals in Olympic history:
Elena Isinbaeva, Russian pole vaulter. World record holder and 2012 bronze medal winner:
"People who support me and love me."
Gabby Douglas, two-time gold medal winning U.S. gymnast:
Jackie Joyner-Kersee, three-time gold medal winning U.S. athlete whose heptathlon world record has stood since 1988:
"This is probably a strange answer, but I'm an asthmatic and I had to have my inhaler with me all the time because I was always afraid I might have an attack. The weather might change wherever I am so I kept it inside my sports bra. I couldn't live without it."
Ian Thorpe, five-time swimming gold medalist, the most golds of any Australian:
"I actually had a bag of licorice that no matter how I felt, whether it was a good swim or bad swim, it was my go-to. So bizarre... probably the worst thing to have as a thing at the Olympics."
Felix Sanchez, two-time gold medal winning 400m hurdler, representing the Dominican Republic.
Sanchez won gold in the 400m hurdles in 2004, but as he was preparing to run his heat at the Beijing Olympics in 2008, he learned his grandmother had died. He "cried the whole day and ran terribly," he says, and did not qualify for the semi-finals. This year he ran with a photograph of himself and his grandmother tucked inside his running vest -- and regained his gold medal.
"I made a promise," he says. "I wanted to win one more championship for her."