Does it take 18 months for twins to be born or nine?

Nine, reason being. A human life take nine month to develop.  The womb of a women can develop more than one life at a time.  

p.s.- it would also take nine months for triplets. 

Nine, give or take. We say nine months, but really, some children are born early, or later than planned. But normally, It takes the same amount of time to create twins, than it does to make just one child. They are both conceived at the same time, so they are growing at the same times, together. If it were to take 18 months, it would be because one twin was created, then the other started creating, by that inference.
Hope this helps!

1. when the tempature rises,your sweat glands and then springs into action making perspiration. some sweat evaporates with your skin taling heat with it.2.some of the blood pumped by the heart travels by your lungs to get rid of carbon dioxide and pick up oxygen.3.saliva produces by, you drinking water which saliva is 98% of. when mucus builds up in your body it also creates saliva as well as, antibacterial compunds.4.when the core body tempature drops, the shivering reflex is triggered to maintain homeostasis. for example: when you get out the pool you start to shiver because it is cold.