Dividing by half

Since we were on the topic of fractions, I taught the boy how to divide by 2 mentally. I told him that if 2+2=4, then half of 4 is 2 and so on. To make certain he has grasp the concept, I went on to ask him what's half of 8, 16, 10, 14, 18 etc. He looked upwards, thought about it and gave me all the right answers. Last night, the hubs randomly asked the boy what was half of 100 and the boy said 50!