Opa's Puzzle

Ok, here is my Opa's puzzle for you: One when coming, two when going. What is that?

Think...think...think...

Ah, OK, I'll give you the answer. It is my second tooth! It has emerged from the lower gum (right side in the front).
So my Opa was very clever to come up with the puzzle. I had only 1 tooth when I came to Jakarta, and I am going back to Singapore with 2 teeth now :-D