What is the easiest way to grind poppy seeds?
You need to acquire a poppy seed grinder (picture #1). It's still not easy to grind a lot of poppy seed — you will need to break out your supply of elbow grease — but it's much more efficient than a coffee grinder. It also does a better job, because it actually crushes the seeds to release all their oils and other goodness, rather than haphazardly cutting apart some of the seeds and not others. (They do make electric grinders [picture #2], but they're aimed at bakeries and the like, not for home use.)

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If you're stuck using what you have, namely the coffee grinder, try adding sugar. It helps keep the poppy seed from sticking everywhere you don't want it to. Naturally, you then need to take the sugar quantity into account when you're actually using the poppy seed.