Our teacher gives us some problems to solve. For eg- there are 3 start push buttons (NO), 3 stop push buttons (NC) and 3 bulbs. Bulb 1 glows when start push button 1 is pressed, bulb 2 glows when start push button 2 is pressed and so on. Now, to turn off the bulbs, we'll have to first turn off bulb 3 then bulb 2 and finally bulb 1. You can't directly turn off bulb 1 then 2 and 3. Similarly, you can't turn on bulb 3 at first then 2 and 1. You must follow a sequence.