One day I was explaining the how the digestive system works then she suddenly asked me how does urination take place I still have not found out can anybody help me

There are a multitude of chemicals that actually make up urine. Urea, a major constituent of urine is actually cell excretions from throughout the body, water from the digestive system ends up in urine as well, and the kidneys and ureter also play a major role, so you can't really name an EXACT place where it comes from.

Basically, blood is passed through a "sieve" to keep in the proteins, then of what goes through the "good stuff" is reabsorbed, and the rest discarded. This mechanism means that the kidneys can discard just about any chemical, even those you've never been exposed to before.