Sometimes, if you are groggily taking a shower in the morning without paying much attention, you may forget whether or not you’ve already washed your hair (Figure 1).
You can, of course, always wash your hair again, but then you wasted both time AND shampoo.
What if the bottle itself could indicate (in a simple and electronics-free way) when it was last used?
Here are two possible ways for a shampoo bottle to indicate when it was last used, without requiring any significant user interaction or complex mechanisms (see Figures 3 and 4).
PROS: Saves time and shampoo.
CONS: May add manufacturing costs and additional product waste.