This animation shows you how the Biuret test works. If the foodstuff contains soluble protein, the reagent turn from light blue to purple. If it stays light blue, then the foodstuff does not contain protein.
Your teacher may give you two solutions marked “Biuret A” and “Biuret B”, but in this animation I have already mixed the two together for you.
The animation shows you that chicken, meat, fish, and beans contain protein. Potato does not contain protein. Protein is also found in eggs and in other members of the pulse family ie peas and lentils.