Have you ever tried to get the last drop of laundry detergent out of the bottle? Leaving the bottle upside down gets quite a bit out, but there's still more trapped. The design of P&G/Tide/Gain bottles makes it difficult to get more out. I could rinse out the rest with a lot of water, but that's actually too much soap for one load. Another way to get the last drop and rinse more effectively is to pry off the cap insert with pliers.
Living in a dorm with shared laundry rooms, I saw how this inefficiency is multiplied. So many students throw away the bottles without trying to get all the soap out. They could be too busy to worry about it, or maybe they just don't know because they don't have much experience doing laundry. Imagine a dozen bottles of soap thrown away, each with over 4 ounces of soap left in it. That's also a dozen bottles thrown away- not clean enough to be recycled even if they had been put in a recycle bin.
My engineering solution is to drill a hole in the corner of the bottle and let it drain into another bottle. Then the empty bottle can be rinsed to use every bit of soap, and then it will be clean enough to recycle. I was shocked that over one semester, I collected enough leftover soap to fill a whole bottle!
