If a molecule started in a fish and ended up in a grass, then the molecule probably followed this path:
The molecule starts in the fish.
By some reason, the fish dies in the water.
The body of the fish is washed to the shallow part of the body of water.
It decomposes and the molecule escapes into the body of water.
A grass grows near the body of the water and absorbs water and nutrients in the nearby body of water.
It draws in water including the molecule that was it in the fish.
