How is it that some people can do this and others can't?!

Hi, I'm 16 and when I was 10, my friend told me that when her sister was in gym class, she did this stretch where she put her legs behind her head. I thought that was cool so I went in my house and the first time that I tried, I did it. Recently in my aerobics class, the teacher was making us do stretches and she told us to do that. I was the only one in the class who was able to do it. I talked to my mom about it and she said that she only knew 2 people that could do that when she was in high school. I don't exercise a lot, but I do whenever I get a chance. Is being able to do that something genetic or what?