Dead Bug

From AI Gemini: (PS - Yours truly does 100 of these a day.) 

The "dead bug" is an excellent choice for core stability because it targets the deep abdominal muscles without putting the same strain on the neck or lower back as a traditional sit-up.

Short and Sweet Instructions:
1. Starting Position: Lie on your back with your arms extended toward the ceiling. Lift your legs and bend your knees to 90 degrees (so your shins are parallel to the floor).
2. The Movement: Slowly lower your right arm behind your head while simultaneously straightening and lowering your left leg toward the floor.
3. The Key: Keep your lower back pressed firmly against the ground throughout the move.
4. Return: Bring them back to the starting position and repeat with the opposite arm and leg.