There’s a tsunami in heaven and you take a small drop of water and append it to the tsunami. Now the tsunami is big enough to destroy all of heaven. The small drop that you appended to the tsunami was just enough to push it over the edge, reaching full destruction capability.
Just after you append the small drop of water to the tsunami, it grows ten (てん) stories high. Before it was only nine, but since you appended that drop, the tsunami is now a full ten stories tall.
Currently there are no similar looking kanji which exist in the WaniKani learning system