Why is waiting time memoryless? (in Stochastic Processes)

[colstat]

I am learning Stochastic Processes right now. Can someone some explain why waiting time is memoryless? Say, if a light bulb has been on for 10 hours, the probability that it will be on for another 5 is the same as the 1st 5 hours.
It doesn't make sense to me, because the longer you use it, the more likely it will break down. No?

 

[mathman]

Waiting time is memory-less for processes that have been defined to be such. As you observed there are processes which may not be.

 

[colstat]

got it, thanks!