Solution: First thing to do is decode the line-dot shapes. Each one can by laid over a 3x3 grid and any place where the line is solid (not a dot), that square is marked. Two line figures "fit" when their grid representations are complementary. For example, in the samples below, the two grids created from the lines are complementary.

Using this method, one can find a pair of complementary lines in each row of the puzzle (In the first row, lines 1 and 3 are complementary, then 2 and 3, then 1 and 3 in the last row). Reading the letters of these 6 figures in order gives "STACK OTHER TRIPLETS". Stacking the other letters, gives a 3x3 grid of letters:

One also is left with the three unmatched line figures, which can be transformed into grids which can be laid over the letter grid. The letters under marked squares for each line spell out a word, reading left to right, top the bottom. The three words are "WATER", "EARTH", and "AIR". The missing fourth is the solution word, "FIRE".