Project - Arduino to know the difference between 2 objects

What about placing a different colour label on bottom of food & water things and using a colour detector.

Along these lines,

http://www.parallax.com/Store/Sensors/ColorLight/tabid/175/CategoryID/50/List/0/SortField/0/Level/a/ProductID/429/Default.aspx

For a somewhat cheaper form of this, you might try using a blue Led and a red Led, and a simple CdS photoresistor for
pickup. First flash one Led, then flash the other Led.