I had this question in my mind for many days.
The main question came to my mind from times ago when I was working on some electronic devices and at that time I had an ac adapter that it had some keys on it which let me to choose voltages from 1.5 to 12 (It was not advanced like those power sources that are used on electronics labs, it was at the size of a laptops power adapter) it had open wires at the end and I used to twist the wire ends of my circuits to the adapter's ones.
In the middle of my work and when I was building the circuit to connect and test with the adapter, suddenly the adapter blown (boom!) I had no idea of what caused this, why would an adapter blow? I had no idea, I just plugged it out of wall fast to prevent further mess.
I now have made my own power source, I had an old samsung phone (which is dead now - rip) it had a charger which had an output of 4.2V and 650mA and I cut the end that connects to the phone and that gave me two open wire for using in my projects.