My DDs are 14 months old and although I get the whole ignoring things idea there is some behaviour I just can't, namely biting and pushing one another. I have tried doing the firm 'no' and shaking my head which seems to work with DD2 - she usually stops what she's doing. However DD1 seems to enjoy getting up to mischief and laughs and points at me when I say no at her (it's very cute but I do manage not to laugh)and then continues whatever it is she's doing, e.g. pulling wallpaper off the wall, hitting her sister! Any suggestions for letting them know there is some behaviour that's not acceptable? Or are they far too young for this? What would you do?