Yes, the spark plug.
It is a very small, and inexpensive, replaceable part, yet without it, the car will not ignite.
Same thing goes for many aspects of life. A relationship for example. Imagine a relationship without the spark, it is like a car without the spark to ignite the engine. The only way to move that car is by releasing the handbrake, and use brute force to push the car from behind.
If your spark plug is good @ your car got the spark, your drive will be smooth and you'd make good distance, and even if you wanna stop for a rest and 1 of the R&R area on your way from KL to Penang, you know that when you get back into the car, your engine will re-ignite easily, and you continue with your journey.
For reference, a Kancil with good spark plug is still much better compared to a Ferrari with no spark plug at all if you wanna go from KL to Penang, or JB, or Kuantan etc, right? What's the point of the Ferrari if you can't ignite the engine? Are you going to push it all the way to JB?
One thing those guys at Transformers got right is the name of their energy source: AllSpark.
Even the robots' life source are called their spark. Without the spark, there can be no life.