Actually, it will eventually open, because as the water in the engine gets hot the heat will transfer through the water in the hoses and eventually get to the thermostat. BUT, then what happens is the thermostat opens very quickly and you get a big pulse of water through, which is hotter than you want because it took a while to get the thermostat open. On my car the first time I put it together I neglected to put the holes in the thermostat. The water temp sender was in the thermostat housing. The first time the engine warmed up I was staring at the water temp gauge, wondering why it was moving so slowly. Suddenly, BAM, it shot from 160 to 220 in a flash, then rapidly cooled down to 180 as it cycled through the radiator. The holes in the thermostat prevent that problem.