Acorns of many oak species are edible.

They've been used as food in many traditions, but almost all need boiling and/or soaking to leach out the tannins. I'm curious to try them, as nuts, [a hot drink](http://allrecipes.co.uk/recipe/33207/acorn-coffee.aspx), or ground into flour for baking. I'm particularly interested in the European Oak (a.k.a Quercus robur, English or Common Oak) as I have several growing over my garden.

I've found plenty of sources on how to prepare them, but little on how to select them. I assume they should be brown to be ripe, but do I need to get up and pick them as soon as they turn brown, or are fallen ones good? Apart from obvious signs that something has started eating them, is there anything I should beware of?

Update:

Even after discarding the floaters I had to throw away quite a few. they were quickest to shell by quartering them lengthways with a sharp knife.
Unroasted ground acorns worked well in place of ground almonds in a biscuit (cookie) recipe, but the coffee substitute has a flavour closer to malty than nutty and isn't great. I may not have roasted the acorns enough, because despite leaching until they weren't bitter, they turned dark brown on drying so I couldn't see how roasted they were.