The butterfly stretch is a great because it can be modified to fit individuals with different levels of flexibility. You will stretch the adductors, the muscles that pull your legs toward each other, which you might know as your "groin". You start by sitting on the ground, knees out to the side, bottoms of feet touching. Depending on your flexibility level, you can decide how close your feet are to your body. You can see the differences in the pictures below. You want to use your hands to push your knees down to the floor until you feel a nice stretch. Hold for 18-30 seconds. Repeat if you'd like.

This view is for the "less flexible" crowd. As you can see, my feet are far from my body.

This view is for the "moderately flexible" crowd. My feet are a little closer to my body now and I'm still pushing my knees down.

This view is for the "most flexible" crowd. Notice my feet are pulled all the way in towards my body.