Have the flyer lay on her back on the floor, arms down at her sides. You (or anyone) should be able to pick up her feet (behind the heels) and her whole body should lift off the floor like a plank. She should be able to stay tight so that when you wiggle her, she moves as one (like a plank), if you try to pull her feet apart (side to side, or up and down) you shouldn't be able to move them-she should stay tight enough the whole time that you can't separate her feet/legs. The only part of her touching the floor should be her shoulders. It's not all core strength (obvi it is a main part of it) but a big part is also body awareness and control. A lot of girls don't realize just how hard they have to work to truly stay tight, and this is a good way of showing them.