Its a thorn...

... in my back tire. That was the diagnosis on the flat in the back. By lunch time the tire was completely down. Since I would be nervous removing the back wheel of a normal bike to find and fix a flat at home on the weekend I basically bit the bullet and got it fixed in town. There are a three bike shops within walking distance of the office: one that caters to "serious bikers" (i.e. prepare to pay a lot), one that didn't have the greatest reviews on Google Plus, and then there was Griffin Cycles. Gave them a buzz, took the bike over, and 4 hours and 26 dollars later the bike is fixed.