Simple ethidium bromide question

[Pascal_Bochet]

Hello
I was wondering why is it that in PCR when you look at the products on a 
agarose gel stained with ethidium bromide, you can see primer dimer if there is 
any but you cannot see single stranded primer (if for instance you omit TAQ or 
dNTP from the reaction.) Is it the size or the fact that it is single stranded 
? In other terms what does ethidium bromide exactly bind to?
Thank you for any insight.
Pascal_Bochet at brown.edu