Alpha hydrogen and hyperconjugation Why do we say that an alpha hydrogen is necessary for hyperconjugation? What exactly is an alpha hydrogen? From what I read online, it is the hydrogen attached to a functional group. Why is that required? 

For example, in $\ce{CH3-CH=CH2}$, hyperconjugation takes place, but I don't see clearly any functional group that would facilitate it.

Alternatively, in $\ce{CH3-COOH}$ it does not take place.

Why is this?
To remember:

Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give an extended molecular orbital that increases the stability of the system. Further information.

The proposed systems to take into account.

Looking at the resonance structures (which is a synonym of mesomeric effect) of the propene


(source: tutorvista.com)  

and acetic acid


(source: nonsibihighschool.org)  

Therefore, in hyperconjugation analysis you need to look at the position of the electron density. Not the vicinal hydrogen. 

Moreover and with the aim to give a complete answer, the hyperconjugation of the acetic acid can be very well understood at here.