kenwood120s:
And the mains
Well you have to be carefull with mains because of the voltage. It is high enough to send deathly currents (which are bugger all BTW, in the order of magnitude an Arduino will consume) through your body resistance.
A car battery cannot do that. You can safely touch the poles. HOWEVER, it is capable of sending very high currents through stuff with much lower resistance than the human body, and that stuff may quit that by violently exploding in a cloud of molten metal. To avoid that, there is e.g. the recommended order of attaching a jumper cable, which minimized the risk of short-cuts when a clamp is dropped.