Grandfathers muscles

My grandfather worked in a blacksmith shop when he was a boy, and he used to tell me, when I was a little boy myself, how he had toughened himself up so he could stand the rigors of blacksmithing.

One story was how he had developed his arm and shoulder muscles. He said he would stand outside behind the house and, with a 5 pound potato sack in each hand, extend his arms straight out to his sides and hold them there as long as he could.

After a while he tried 10 pound potato sacks, then 50 pound potato sacks and finally he got to where he could lift a 100 pound potato sack in each hand and hold his arms straight out for more than a full minute!
I looked astonished, then with a grin he said:

"Next, I started putting potatoes in the sacks."