How to clean potatoes<br>Start with clean hands. Be sure to wash your hands first using an antibacterial soap, and to rinse them well. You don't want to transfer any bacteria to your clean potatoes.
You can dirty your spuds if you don't wash your hands
The context says you can transfer bacteria from your hands to potatoes if you don't wash them. The system could likely not understand that spuds = potatoes. 
( z1 possible :ARG1 ( z2 dirty :ARG0 ( z3 you ) :ARG1 ( z4 spud :poss z3 ) ) :condition ( z5 wash :polarity - :ARG0 z3 :ARG1 ( z6 hand :part-of z3 ) ) )