These girls thought they were funny posting this photo on Facebook. They were learning about how to use a thermometer when they decided to stick in their mouths. The thing they didn't know is that different thermometers have different uses. The thermometers they used are "rectal thermometers," meaning they are used in a person's rectum. That's not a very pleasant thing to place in your mouth. They probably deleted the photo afterwards.
Sadly, due to how the Internet works, a photo or a post is never truly deleted. Anything a person posts online can easily be achieved in several places. Some of these archives are automatically created, such as the one run by Archive.org, while others could simply be screenshots that people have taken before the other person had the chance to delete the post.