My daughter is in 2nd year of Kindergarden. Every night she has to solve a set of math problems under a name of "Kumon", 公文，apparently a Japanese teacher. It is very popular in China. All kindergarden use it, maybe more popular than Lego.

 

She is doing adding right now. It goes like this: 1+1, 2+1, 3+1...it take her almost 1 month to get to 100+1. Then all of a sudden, it escalate to 200+1,  400+1...1000+1. She looked at 1000+1 dumfounded and then bursted into tears. After an hour or so she wrote down: 0001.