I have a test tomorrow and this might be a question on it. I do not know the answer and I have no idea how I would draw it out if I had to. The question is...
Describe the locus of the centers of all circles of the same size that are tangent to a given line.
Consider a person riding a bike. The circle is the wheel, the line is the floor. The center is the bolt with which the wheel attaches to the bike. How will the bolt move if the floor keeps perfectly flat and even? Will it's height relative to the floor change? As the bike moves then, what will the bolt describe? Making a drawing should prove very useful.
ADD It might also be the case you're being asked to consider both sides of the line. Then you'll just get a mirrored version of what you're getting from the above.