She's middle-aged and gray-haired and stays up all night. Still, Heidi the cross-eyed opossum is Germany's biggest star, and she has not even made her debut at the Leipzig Zoo. The 21/2-year-old opossum has a Facebook following of more than 111,000 fans, has sparked a popular song on YouTube and will soon star as a stuffed animal toy. However, the public will not get its first glimpse of Heidi until July, when a new exhibit opens.

The zoo had no idea that the cross-eyed opossum would become famous when Heidi arrived from a Danish zoo in May along with her sister, Naira, and a male opossum. Demand for information about Heidi has been so high that part of the zoo's Web site answers Heidi questions, including why she is cross-eyed.

Zoo officials believe that Heidi's crossed eyes might be the result of a poor diet when she was young. Poor vision is not much of a problem for Heidi, the zoo said. As nocturnal animals, opossums rely heavily on their sense of smell instead of their sight to get around.