I asked Mrs Didge about this. She knows a lot more about anatomy than me. She said it depends on the gender. In her case it's her strength of character. In my case it's my mouth.
Ummm I agree with Mrs.Didge, strength of character is pivotal in making you a good person. Mouth too is equally vital. Yesterday I was reading and very interestingly i learnt that the strength of a muscle can't be measured to perfection.