Affordances:

Good example:

One of the limitations of having a water bottle this big is that it might be too difficult to hold the entire bottle in one hand comfortably. Assuming that someone actually needs to carry around this amount of water, the loop that is provided communicates to the user that a few fingers can be placed into this slot to carry the water bottle.

Poor example:

Even though I walk back to my apartment everyday, I still sometimes get confused as to exactly which way I should swipe my card into this device to unlock the front door. The device does a good job of communicating that a card can be inserted into a slot, but it fails to communicate how the card should be oriented while swiping. To make matters worse, other areas in Harvard have the same exact device with the slot blocked off, but expect users to tap their card on the device. Finally, there is an additional black device underneath this one, which suggests that one could tap an ID card here to gain entrance and unlock the door. This device, however, does not function.

Redesign:

Credit card swipe machines do a pretty good job of letting the user know how to wipe the card. I’ve borrowed a similar tactic to make it clear that the user should swipe with the back of the card facing right (thought it might not be the clearest in this drawing due to my poor drawing abilities). In addition, the bottom device has been completely removed so as to clarify that there is only one device which controls whether the door opens.