It's an echo!

Apparently Indigo got a big kick out of this one.

牙医检查病人的口腔：你的牙上有个大洞!有个大洞。病人：是有个洞，可你也不用说两遍呀。牙医：我只说了一遍。那是回音。

The dentist was examining a patient's cavity and exclaimed, "There's a ginormous black hole on one of your teeth! ZOMG!"A BLACK HOLE!!! ZOMG!"The patient replied, "There is a hole,but you don't need to say it twice.The dentist said, "I only said it once.That was an echo."