Sliding Knee Tucks
Do not try on the carpet!
Best-suited to a smooth surface, this is great for both arms and core. The tucking exercises your core and strengthens lower back muscles at the same time.
It requires a great deal of upper-body strength as well.
Start in the push-up position then slide your feet to below your butt. Bend your knees toward your chest and keep your abdominals tight.
One new exercise each and every day, 365 days of the year.