I swear there is something in the childhood handbook that says when Mum takes you out to the store, you have to be the worst little hellions there are, the thought being that Mum can't take us anywhere so we can spend all our time at the park or playing somewhere.... not a bad rule unless you're the mum.
Child A - the encourager.
Child B - the do-er
Child B does something to irk Child A... then Child A laughs until Child B does something to make mummy so mad.
(I love my children dearly but they are driving me to the brink...)