CREATE TABLE table_22808 ("Episode title" TEXT, "Subject" TEXT, "Music by" TEXT, "Performed by" TEXT, "First aired" FLOAT)

If skeletal system is the subject when was it first aired?
SELECT "First aired" FROM table_22808 WHERE "Subject" = 'Skeletal system'
SELECT "First aired" FROM "table_22808" WHERE "Subject" = 'Skeletal system'