It's actually pretty easy to be electrocuted, it's just about how much of an unlucky sod you are
With that said, a power supply can be really dangerous
yep, it's just that the human body has quite the good resistance
the human body can only take about 25 - 30 mA (a really small amount) of current before the heart can fibbrilate and cause in death, so therefore, a 9v battery should kill us, knowing that it creates a couple kilojoules of energy.
however, the human body has about 3k ohms of resistance, (1k when wet) meaning that when you hold your finger across a battery, it does nothing, but when you put it on a wet spot, like let's say, your tongue, it kinda tingles
whatever it is, don't be an idiot with high voltages