Body parts

I read that when mentioning a body part we are to use definite articles (el, la, los, las) instead of the possessive adjective (mi, tu, su, etc.) but one of the examples on the site says "Tengo dos lápices en __*mi*__ mano," Why is that, and when should I use the possessive adjective when referring to body parts? I also hear the possessive adjective used when I listen to spanish songs on the radio.