If you've got kids, then you know they love to climb on you like you're a human jungle-gym.
Well, because of this, I got a great core workout... and completely by accident.
I was staying at a hotel and my kid was with me, (4 years old). Anyway, the hotel room had 2 queen sized beds and my kid loved jumping from one bed to the other. The beds were about 4 feet apart, and I layed across the gap (face down)... stiffening my body like a "bridge".
Well, my kid saw this and proceeded to climb across, over, and under me... for about 10 minutes straight...
I was sore as hell the next day.