### Instruction: What are the primary features of a gravity hill given the following description?
### Input: A gravity hill, also known as a magnetic hill, mystery hill, mystery spot, gravity road, or anti-gravity hill, is a place where the layout of the surrounding land produces an optical illusion, making a slight downhill slope appear to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill against gravity. Hundreds of gravity hills are recognized worldwide.
### Response: The gravity hill optical illusion is defined by having the following characteristics:

1. A downhill slope will appear to be an uphill slope
2.  Objects appear to be rolling uphill against gravity