I left the gym yesterday with burn marks and bruises down my arms. Ava and Georgia (my 10 year old twin daughters) got very upset when they saw me. “Did someone hurt you?” “Were you in an accident?!” My explanation sounded strange: Mommy is squatting holding a 40lb bag of sand and then flipping it up on to her arms and swinging up on her toes. I showed them the movement with flair to try to get...