my helper today gang, this is tinnie lim. she is the lady in that energy chapter who is pulling the bow and arrow back. tinnie was my student at city college in san francisco in 1980. got into conceptual physics, it spurred her on. today, she is a design engineer at jet propulsion labs in california at pasadena, and she is working on these fly-by missions and all that stuff and right now she is doing what, you are making a pod to fly in the next space shuttle, and that's what she is doing, okay? tinnie, hold these up and then tell me which of those two has the most mass? they feel the same to me. these are the same weight gang, okay? but i want you to do something else tinnie. i want you to hold it at the midpoint, and then rotate it like this. and now try this one. any difference? this is hard to rotate. very much hard. take them both and do it. like something up here? yeah, you can really feel the difference. let's try this. put this in your hand, flip that like this, you do like this, flip it. okay, now change gang, now cha