Answer:
What is easy to spot but hard to find?
I may seem real but it always turns out I was never there in the first place... you only see me during a certain resting stage.
I start new then become old. Start clean but be one dirty. I usually start big then become little.
I run through hills. I veer around mountains. I leap over rivers. And crawl through the forests. Step out your door to find me.
I can be sweet or sour. I do not rhyme with any other word
I tell people to be careful when handling boxes.