Answer:
What has no hands but might knock on your door, and if it does you better open up?
I always run but never walk, I sometimes sing but cannot talk, No head on which a hat to place, You always look me in the face.
A precious gift, yet it has no end, no beginning, and nothing in the middle.
Once you know it exists, it no longer exists. What is it?
Before Google, we actually had to physically look things up in this building full of knowledge.
Those with eyes bigger than their stomach will definitely leave the restaurant with one of them. It is named after a house pet as well.