Why do adults tell their kids not to smoke when they do it themselves?

i think its really stupid one of my friends called laurie* who i dnt talk to no more her mum smokes and when laurie tried a ciggie cuz she was stressed her mum shouted at her for it and i thought that was dead stupid although i kept it to myself but still even my dad agrees its not just that loads of adult smokers tell their kids not to smoke n its silly ive always thought smoking is dangerous n scary loads of ppl in my school do it n i find it gross it dosent make them look cool at all it makes them look stupid soo why do people start smoking specially teens