Silence.

When a female is silent, millions of things are running in her head.

That's so true. Well, at least applicable for me.
There can be many reasons for me to be silent.

I might be angry.
I might be sad.
I might be happy.
I might be shy.
I might be thinking.
I might be dreaming.
I might be annoyed.
I might missed what you say.
I might not want to talk to you.
I might be answering you in my mind.
I might be deaf/mute for the time being.

You never know. ;)