Answer:
I can be simple or I can be complex. I can be found in this riddle or in everyday life. I can be shapes or even colors.
What is easy to get into, but hard to get out of?
What word has kst in the middle, in the beginning, and at the end?
Poor people have it. Rich people need it. If you eat it you die. What is it?
I have four fingers and a thumb, but I'm not living. What am I?
No matter how little or how much you use me, you change me every month.