When I studied retail at TAFE we were told to treat the customer as if they were a 5 year old.. Meaning that you shouldn't assume that they will read something as you see it.. 

We had to do an exercise where we had to write directions for boiling a kettle and it had to include an instruction to fill the lettle with water as you couldn't assume that they would automatically know how to do something.. This is also why they have those tags on electrical items that say "Do not use in Bath" etc in case someone attempts a refund and says "Well it didn't say I couldn't do it"