Mooncakes have a thin crust and can have a variety of fillings. This one was filled with a mixed of lotus seed and red bean paste. The duck eggs inside symbolise the moon (sorry, this photo is a bit rubbish).
The story goes that they were used in the Ming revolution to send messages to other revolutionaries concerning the overthrow of the Mongolians. Messages were hidden in the symbols on the crust, and could only be deciphered by taking a set of four mooncakes, cutting them into four, and assembling the 16 quarters. The revolutionaries would then eat the cakes, destroying the message. They are dense, and pretty heavy going. I certainly couldn't eat a whole one, even though they are only about the size of a picnic pork pie. The sweet paste filling tastes OK, but the egg is a bit cloying.