Mix soot from the stove with talc. Let the child breathe on any finger and squeeze it to a sheet of white paper. Sprinkle this place cooked black mixture. Shake the paper to blend well covered the land, to which was attached a finger. Powder residue ssypte back into the jar. On a sheet remains clear fingerprint.
The reason is that we have to be sure your skin is a little fat from subcutaneous glands. Everything what we touch leaves inconspicuous trail. And made us a mixture adheres well to fat. Thanks to the black soot it makes visible imprint.