If they become too easy for you, there are so many variations to mix it up- from putting one leg in the air to adding weights.
Here's what to do:
Lie flat on the ground then bend your knees so that your feet are about hips distance apart and flat on the floor.
Keep your head, neck and upper back on the group.
Using your core, press your thighs and glutes up towards the ceiling and squeeze all of your muscles.
Slowly lower to starting position. Repeat 15 times.