FACTUAL RIDDLES XLI

From Reader's Digest
Contributed by Philip Spitzer

   When our washer broke, My dad took our family's clothes to the laundromat. He was finally ready to go home, arms laden with our neatly folded laundry, when he noticed a sign on the glass door that said "step on."
   He stepped on and off the doormat several times, but nothing happened, so he pushed the door open with his foot and made his exit. Glancing back at the door, he realized why the door had not opened when he stepped on the mat. It was not due to any mechanical or electrical problem. Why hadn't the door opened?

Answer:

From the outside, the sign on the door said "no pets."