word play

A post of cafenowhere 's reminded me of a sign I saw when I was driving the healing angel to school. (Indeed, it was on that journey that I discovered the brakes were going to need fixing sooner rather than later, and hence the depositing of the car with the mechanic and the walk home in the rain.)

exit only, the sign said, but I saw an S in there, somehow, sneaky snaky S, insinuating in, active imagination, poor vision, I don't know! Anyway, add an S in the right place, and the message becomes...