On a hot summer day, Holmes and Dr. Watson went on a camping trip. After a good meal and a bottle of wine, they lay down in a tent for the night and went to sleep.
   Some hours later, Holmes woke up and pushed his friend. “Watson, look! What can you see in the sky?"
   Watson replied, “I see millions and millions of stars.”
   “What does that tell you?” asked Holmes.
   Watson thought for a while. “First, it tells me that we will have a beautiful clear day tomorrow. Second, it tells me that the time is about twelve o’clock. And…”
   “What else?” Holmes asked.
   “Well, it also reminds me that the world is so big and we are so small. What does that tell you?” Holmes said with a grin on his face, “You silly guy! It only tells me that someone has stolen our tent!”

已知空间四边形ABCD，AB=AC，DB=DC，E是BC的中点．求证：BC⊥AD．

示例：屈原——中国最伟大的浪漫主义诗人之一，创造了“香草美人”的传统审美意象。代表作《离骚》是中国古代诗歌史上最长的一首浪漫主义政治抒情诗。