I connected two LEDs in series.
The voltage between power and ground was 4.94 volts.
The voltage across the green LED was 2.44 volts.
And the voltage across the red LED was 2.49 volts.
So, almost all of the voltage was used up!
The LEDs were not burning out without the resistors.
Because each LED had its own resistance.
How can I measure the resistance of an LED?