What geeky hobbies can you completely lose track of time while doing and why?

Geek Question of the Day: We all have at least one of these, so today we thought it would be fun to ask you; What geeky hobbies can you completely lose track of time while doing and why? As usual, anything goes for this, even if it isn't considered 'geeky'. Another shout out to Scott Anderson for this one!

Oh and just a little note that this is our 2,500th question!!!

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