CREATE TABLE table_16301 ("#" FLOAT, "Air Date" TEXT, "Challenge" TEXT, "Winner" TEXT, "Test-taker" TEXT, "Passed?" TEXT)

What episode aired on 18 April 2007?
SELECT MIN("#") FROM table_16301 WHERE "Air Date" = '18 April 2007'
SELECT MIN("#") FROM "table_16301" WHERE "Air Date" = '18 April 2007'