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](http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch04/ch4-3-2-1.html).


**The proposed systems to take into account.**


Looking at the resonance structures (which is a synonym of [mesomeric effect](http://goldbook.iupac.org/M03844.html)) of the propene


[![Resonance structures of propene](https://i.stack.imgur.com/hRThG.gif)](https://i.stack.imgur.com/hRThG.gif)  

(source: [tutorvista.com](http://image.tutorvista.com/content/organic-chemistry/hyper-conjugation-propene.gif)) 


and acetic acid


[![Resonance structures of acetic acid](https://i.stack.imgur.com/06SSF.png)](https://i.stack.imgur.com/06SSF.png)  

(source: [nonsibihighschool.org](http://nonsibihighschool.org/intbasch9_files/aceticFC.png)) 


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](https://chemistry.stackexchange.com/questions/27799/hyper-conjugation-in-acetic-acid).


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