Water hose is good, but we just put sheets over the windshield and hood- it seemed to do the trick. The problem is, the cats like the warmth after the engine has just turned off, so if you can get him another nice snuggly warm spot he'll most likely leave the car alone.

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I am certain of nothing but the holiness of the heart's affections and the truth of imagination. What the imagination seizes as beauty must be truth - whether it existed before or not.
-John Keats