SAN FRANCISCO -- San Francisco is seeing a spate of mini cars being tipped over in the middle of the night, but police don't know if the acts are a protest or a prank.

The San Francisco Chronicle reports (http://bit.ly/1p74YcK ) that two Smart cars were found turned on their sides less than a mile apart early Friday.

The incidents came two months after four other Smart cars were toppled in another part of the city.

Smart cars are small, eco-friendly vehicles that weigh about 1,800 pounds, which is about 1,200 pounds less than a Toyota Prius.

San Francisco police spokesman Gordon Shyy says police are investigating the episodes as vandalism but that no arrests have been made.

The Chronicle says car-tippings also have been reported in Amsterdam and Canada.

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Information from: San Francisco Chronicle, http://www.sfgate.com