JellySock, Twitter asked:
Are all cells replaced in the body over time?
Richard Adams - I don't know what happens to cells in the teeth. I do know that cells in the lense in the eye, for example, don't regenerate - we keep the same population of cells throughout our lives. Tattoos - well tattoos do in fact lose their resolution over time, they become a little blurred over time. So I think the dye that becomes embedded within the tattoo gets dispersed as the cells in the dermis are replaced.
Chris Smith - They can also get dispersed when certain body parts grow or shrink, so Cheryl Cole needs to be careful because her posterior which has a rather large rose on it could shrink or expand.