When sticks of kindling wood are vigorously rubbed together, heat is produced by contact with air, and a spark of fire appears.
(sticks of kindling wood; are vigorously rubbed together; ) (heat; is produced; by contact with air; T:When sticks of kindling wood are vigorously rubbed together) (a spark of fire; appears; )
sticks of kindling wood heat a spark of fire
are vigorously rubbed together is produced appears
by contact with air T:When sticks of kindling wood are vigorously rubbed together