CREATE TABLE table_18953 ("Ep. No." FLOAT, "Title" TEXT, "Director" TEXT, "Written by" TEXT, "Original Air Date" TEXT, "Production No." FLOAT)

Who wrote the episode called 'mercury falling'?
SELECT "Written by" FROM table_18953 WHERE "Title" = 'Mercury Falling'
SELECT "Written by" FROM "table_18953" WHERE "Title" = 'Mercury Falling'