user: Question: If Skeletal system is the subject when was it first aired?
Columns: ['Episode title', 'Subject', 'Music by', 'Performed by', 'First aired']
Types: ['text', 'text', 'text', 'text', 'real']
Sample row: ['The Body Machine', 'Nutrition and digestive system', 'Lynn Ahrens', 'Bob Dorough and Jack Sheldon', 1979.0]
SQL:

assistant: SELECT "First aired" FROM "Table" WHERE "Subject" = 'Skeletal system';