To celebrate planet Earth and Earth Day, NASA has called on citizens of Earth to take a #GlobalSelfie. But, what is a #GlobalSelfie? Put simply, it's a selfie. As NASA explains it, a #GlobalSelfie is a self-taken portrait of Earth's inhabitants. Snap a selfie and post it to one of five social media networks, including Twitter, Instagram, Facebook, Google+ and Flickr, with the hashtag #GlobalSelfie. Anyone with a camera and an internet connection can participate. When you take a picture of yourself and your friends for NASA's #GlobalSelfie Earth Day celebration, make sure to include a sign (NASA has several available) telling NASA and other viewers where you are on the planet. NASA suggests trying to get a glimpse of your surroundings in the picture: "Get outside and show us mountains, parks, the sky, rivers, lakes -- wherever you are, there's your picture." Of course, you don't have to use a NASA-designed paper sign. You can get creative: spell out your location in the sand, or by arranging rocks or sticks. "Once those pictures stream around the world on Earth Day," NASA explained on their website, "the individual pictures tagged #GlobalSelfie will be used to create a mosaic image of Earth -- a new 'Blue Marble' built bit by bit with your photos." Below are some of the best #GlobalSelfie pictures posted to Twitter so far: Hi @nasa! We are at #LosCabos celebrating the #EarthDay! #globalselfie #bcsmx #travelgram #seatrek @caboexpeditio... pic.twitter.com/vw9DocTxok— Alex Gil (@Gil_A_lex) April 22, 2014 Going on a #nature walk to celebrate #EarthDay, but first let me take a #GlobalSelfie. #selfie #OnlyYou pic.twitter.com/pq4dGKfAIr— Smokey Bear (@smokey_bear) April 22, 2014 #GLOBALSELFIE Isn't a selfie but, its a good picture, from Simon Bolivar park, Bogotá Colombia. pic.twitter.com/fzBIJG8ffC— Patricia Arboleda (@Parboledaa) April 22, 2014 Today Earth day and #GlobalSelfie ! Thrush is early this year in #Toulouse pic.twitter.com/3wdLfEmNA2— Isabelle (@idariane) April 22, 2014 Another #IceBridge #globalselfie. This time from downtown Kangerlussuaq, Greenland. pic.twitter.com/2u3akWMKoL— NASA ICE (@NASA_ICE) April 22, 2014 Jamie Nehrir getting SAGE III/ISS ready to study ozone from @ISS_Research. Launch set for 2015. #GlobalSelfie pic.twitter.com/K8d0mg6P1w— NASA_Langley (@NASA_Langley) April 22, 2014 #GlobalSelfie @NASA pic.twitter.com/o2DbLsQvZe— Lord Vader (@iLordVader) April 22, 2014