Those Ghastley Thorns.

Over the past three months I have replaced more tubes in my bike tires than I have in an entire lifetime.  It appears that there are these tiny plants that grow in arid areas like dirt trails and in pavement cracks that have rock hard thorns just long enough to puncher the inner-tube of a mountain bike tire and small enough that you can not see them without examining the tires really well.  I replaced both front and rear tubes yesterday, and picked up two self sealing tubes today to carry as spares.  Then I went for a 42 mile ride and not a single leak occurred while on the road.  If the tires are still inflated to 80 pounds tomorrow morning I missed the thorns.