When you read the specifications of the Arduino UNO they will tell you it should be powered by 5 volts and consumes about 40 mA without any external components connected.
Each of its outputs can deliver about 40 mA, and in HIGH state it should provide nearly 5 volts.
So you might come up with the idea to power one Arduino by another one.
pinMode(10, OUTPUT); digitalWrite(10, HIGH);(You could also power the second one by connecting their 5V-pins together which you will find somewhere next to the two ground pins as well as on the ISCP connector.)
Actually, when you are doing it that way, there will be a voltage drop, and the second Arduino does not get the full 5 volts.
As we went so far we can even continue to extend it to a daisy chain:
all the following Arduinos get less power than the preceeding one, but even number four will work.