To "walk a fine line" means to be very careful that you do not make a little mistake that would cause a problem--ie., I walk a fine line when I talk to my children about how to raise my grandchildren --I must not "interfere" in their parenting, but I want to share my thoughts about what I "approve of" and "disapprove of." So am am careful NOT "to cross the line" that makes me an "interfering grandparent."

This is what you might tell your child if they are testing your patience. You are walking a thin/fine line. You are going to get in trouble if you are not careful! Pierce was walking a fine line with his comment because it was a little rude. It can be used when you are trying to find a balance between 2 extreme behaviors. He was walking a fine line between being funny and inappropriate. His comment was a little bit funny, but was very close to being rude/inappropriate.

It means that you need to get things precisely right. There is no margin for error. A "fine line" is a thin, narrow line. You might draw it with a "fine-point pen." The line you need to walk on is narrow. It is not wide. Bad things will happen if you step even a little bit off the "line." I once walked along the edge of a "fin" in Arches National Park. The level part was about three feet wide. There was a thirty-foot drop on each side. I needed to "walk a fine line" or I would have fallen off. "Fine" has more than one meaning. The most common meaning is "excellent, high quality." "Toyota makes fine cars." However, another meaning is "small." "The contract was printed in fine print." (The letters were tiny). "She has fine hairs on her arm." (You can hardly see them). "Is this coffee grinder set for a coarse grind or a fine grind? (Are the coffee grounds big or small?)