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]