Question on circular saws

[fredtoast]

i have owned a cirular saw for years. I have not ysed it very much, but I have changed the blade before when I wnated to cut some metal fence posts. Best I can remember all i did was stick a screwdriver into the teeth so the blade would not turn then unscrew the bolt holding the blade on. So last night when I went to change the blade the bolt would spin the motor instead of unscrew even when the blade was locked in place.

So I went and got my dads circular saw and the exact same thing happened with it.

So WTF am I doing wrong?