@Hayleyotp Hey look! On oldish post. Well, for one, why are you pressing your thumb against a plastic knife in the first place? And if you want an explanation (assuming that you are talking about the knife turning yellow) than it is simply your oils of your skin rubbing off and staining the plastic. (assuming you are talking about your thumb turning yellow) Same as stated above, just seeping back into your fingers, or just the fact that when you apply pressure to your skin it causes a temporary skin discoloration.