If you save the empty plastic containers from butter, yogurt, etc, use a permanent marker to put an X or some other mark on the lid so you can tell at a glance what really is butter and what is probably leftovers.
When I purchase margarine, I buy the one that comes in two small containers, 1/2 lb. in each. They make great containers for leftovers. I just write on the lid what it contains with a black sharpie. Works great.
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