Summarize this article:

Very small scratches that need to be touched up quickly can actually be repaired using only a walnut or pecan. Begin by breaking the meat of the nut open so that the nut's oils are exposed. Gently rub the nut back and forth along the scratch in the wood surface. The oils in the nut will naturally fill in and darken the scratched area, lending the exposed raw wood a finished look. This technique works well for quickly minimizing the appearance of small surface imperfections.

Summary:
Break open a walnut or pecan. Rub the broken nut across the scratch.