Order of electric components
In a series circuit if we permute two different bulbs then each bulb
maintains its brightness.
In a series circuit, modifying the order of connection does not affect
the way electric components operate.
Number of passive components
If we add a second bulb in a circuit including a bulb and a battery
then the brightness decreases.
The brightness decreases even more if we add a third.
The result would be the same by adding a motor or a resistor instead of
The more a series circuit includes passive components, the
more its bulbs get brighter.
Burnt out bulb or unscrewed bulb
If a bulb is burnt out or is unscrewed in a series circuit including
multiple bulbs then all bulbs stop to shine.
If a bulb burns out then the filament is broken and when a lamp is
unscrewed there is no more contact between the bulb and the socket.
In both cases the circuit is open and the electric current can not flow.
Conclusion: In a series circuit if a bulb is burned out or unscrewed
other passive components do not work anymore.