[FabHK]

And a related counterintuitive fact (again, assuming 10 minutes):

1. If you pick a block randomly (uniformly), its average length is 10 minutes.

2. If you pick a point t0 in time randomly (uniformly), the average length of the block you're in is 20 mins (and the average length from t0 to next block is 10 mins, and the average length from previous block to t0 is also 10 mins (and, needless to say, 10+10=20...)).