Binary number 4

Interesting nerd story:

"A couple asked me to teach a five years old some basic programming knowledge.  When I asked whether it's too early to do so. The mom, 'maybe my son is my genius." In order to stimulate this genius to understand logic, I tried to teach him binary arithmetic, just like teaching little kids how to count.  So I ask him to use his fingers to represent binary numbers.  For instance, one is 00001, so he raised only the thumb.  Two is 00010, so he raised only the first finger.

Things went well until the counting reaches number 4.  Since then the parents prohibit me from being close to their son."