The sound of rain hitting your windscreen can mean dicey road conditions and snags in traffic. According to XKCD's What If, it can also mean the utter destruction of your car's windscreen by the force of those rain droplets hitting the glass. Lesson learned: don't drive at speeds of Mach 1.5 in a downpour if you want your glass to survive.
As XKCD's Randall Munroe explains, at Mach 1.5 (or nearly 1,000 miles per hour), the droplets hitting your car's windscreen will smoosh out into sideways-aimed water jets doing an astounding Mach 18. The force of these jets will simply erode your windscreen glass—meaning those 1,000 MPH droplets will now be hitting your face. Ouch.
So, slow the hell down when you're driving in the rain. Keep it at, say, a sensible 750 MPH. Your glass will thank you. [xkcd]