boil down
  煮濃,熬濃,壓縮
  boil down
       v 1: be the essential element; "The proposal boils down to a
            compromise" [syn: reduce, come down]
       2: be cooked until very little liquid is left; "The sauce
          should reduce to one cup" [syn: reduce, decoct, concentrate]
       3: cook until very little liquid is left; "The cook reduced the
          sauce by boiling it for a long time" [syn: reduce, concentrate]