We've all been taught in primary school how we're supposed to use *'an'* instead of *'a'* when we talk about an object whose name starts with a vowel, in its singular form.  

> 
> -> **An Apple**   
> -> **An Ostrich**  
> -> **An Elephant**
> 
> 
> 

But when we refer to one *university, universe* or even adjectives like *useless*, we use *'a'* instead of *'an'*.  

> 
> -> **A University**  
> -> **A Useless Boy**  
> -> **A Unique sight**
> 
> 
> 

Why is that so ? I get the fact that we use *'an'* for words which start with consonants occasionally, for words like *'Hour'*,*'Honor'*, because the *'H'* is silent. But what of the exceptions in the case of words starting with *A,E,I,O,U* ? Or is the just for certain words that start with *'U'* ?