Scissor kicks are a great way to tone not only the core, but the legs. Throughout this movement, you're reaching long through the balls of your feet, keeping length in each leg the entire time.
While on your back, you have the option of lifting the head or keeping it lowered on the mat. Tucking in the abs, extend both legs long to the sky. With the exhale, allow one leg to lower to the mat as much as you'd like while the other is pulled gently to the torso. With each exhale, you switch those legs.
Imagine them moving close together like a pair of scissors, almost touching but not quite. This posture is a great segue to many others- The Bicycle, Single-Leg Extension, Criss Cross, and many others we'll get to.