Temperature?

We called some catholic school girls in uniform as they passed the booth. 

We instructed them that we knew it was this payphone.  We were located three blocks North, but we knew this booth was under a billboard that flashed the time and temperature every 20 seconds. 

"Could you please tell us what the temperature is?  We need to know".  The girls left the booth and walked out in the parking lot till they could see the billboard, then returned with the value. 

Sounding disappointed, we got them to go out a second time because we wanted Celsius temperature, not Fahrenheit.  We then asked to speak with the other girl, and they switched... then suspiciously started asking how we knew there were two of them.  Not wanting to blow our location cover, we made something up that was stupid, but they bought it. 

When we started complimenting them on their clothes, they knew it was a prank and started giving the finger randomly in the air, hoping wherever we were, we'd see it.