The lemon squeezer is a challenging ab exercise that is scalable for any level. All you need to perform this movement is some space on the floor and maybe a yoga mat if you need some padding beneath you.
Start sitting on the floor with your knees bent and feet planted. Sit up nice and tall and engage your core by pulling your bellybutton in towards your spine.
Raise your feet to hover above the ground and hold your arms out to the sides. Slowly extend your legs out while simultaneously leaning your torso back towards the ground.
At the bottom, your body should be extended with your legs straight and feet hovering a few inches above the ground. Your lower back should be pressed into the ground while your shoulders and arms hover as well. This is the bottom of the position.
To come up, think about squeezing your abs to bring your torso and legs back to the starting position. Pause for a moment at the top. This is one rep.
If you do not yet have the core strength to do this version, you can do a scaled version by keeping your hands on the ground behind you. Lean back, tighten your core and complete the movement as described above, but using your hands for a little bit of assistance.