This super exercise works the abdominals and doesn't strain the back like sit ups - if done correctly.
Begin with both feet flat on the mat and knees bent. Put your hands on your lower abdominals. Lift one leg up into the air and check your tummy muscles don't bulge upwards.
Place the foot back down and repeat on the other leg.
When you know you can do this without the tummy bulging outwards, then keep the first leg in the air and try lifting the second leg up too!
Most people when they do this will feel the tummy bulge straight away. If you do, and you can't keep them pulled flat, this means your abdominals are not yet strong enough.
To make lifting the second leg easier, try bringing the foot in towards the bottom first, before lifting it.
The aim is to keep your abdominals flat as you lift the legs and to not let your back arch away from the floor.
Let me know how you get on!