heres another idea
when you get home get last site visited by your dad loaded on screen
and then run downstairs asking dad if he can fix your computer, you got a porn site on it, and you didnt even go there
he will think he forgot to close or exit site
another idea is for him to get his own computer & leave yours alone
another idea is to disconnect your network connection when you are done before you go, and reconnect it when you get home
if dad asks about why computer dont work, play dumb and say you will try to fix it, and they reconnect it add the child lock, tell him you got it to work, but now you cant get on some sites you used to
