Why do we act like idiots?

Why is it that some people just do it for you? I mean they only have to walk in the room and you go to mush. One word from their mouth and you are like putty in their hands - all weak, heart racing, knees trembling etc etc. Ok I know those are the trademark signs of something we all know as fancying someone but why do those things happen when certain people walk into a room or for that matter speak to you on the phone? I had one of those stop you in your tracks moments earlier. Someone I wasn't expecting to ring called me up. I just looked at the phone in horror. How could I answer it? How could I talk to said person and hold a meaningful conversation? I wasn't going to answer the phone at all, just ignore it and hope the person went away but I had been texting that person so they knew I was in reach of the phone. I didn't want to appear rude so I had to answer it. Texting is ok as you don't have to speak just send words and symbols to one another - back and forth like a game of tennis. Speaking is an altogether more scary prospect. Having to formulate sentences when your mind has gone to mush and all you want to do is giggle is not at all easy. To appear grown up and normal when you feel anything but, not an easy task. So I pressed the green answer call button and tried and failed miserably to act like an adult who wasn't embarrassed or nervous or giggly or any of the other things I was feeling. Of course now I am on a high, floating on cloud nine. Better than taking some illegal drug!!!!! I feel amazing now, all thanks to one telephone conversation, which wasn't even that good a conversation. So my question to you out there in cyberspace is why do we act like idiots when someone comes into our lives that we like or fancy? What is it that makes sensible adults turn into silly idiots? Any ideas on that one?