rhinoceros VS hippopotamus

My kids asked me to read them a book. So when i read till the page Animal in Grassland, there was a rhinoceros photo.

Yang:"Oh... hippopotamus!"
Jing:"No, it's rhinoceros!"
Yang:"No, you are wrong. It's o!"
Jing:"No. Mummy tell Yang what is it?"
Mummy:"Yang, Jie Jie is right. It's a rhinoceros. See, rhinoceros has horn on their face but not hippopotamus."
Yang:"I learn from school. Teacher said it's hippopotamus."

Mummy... Oh! So, he did learn something from school ha! I thought he just play only! hehe...
Em... What should i do... I search the whole book but can't find the hippopotamus picture. Ok, i surf on-line.

Mummy:"Yang, see here is the image of hippopotamus." Compare with the book, " See, the different between these two animals. Are they the same animal?"
Yang:"(took him 10 sec to compare.) Em... not the same animal."
Mummy:"This is rhinoceros and this is hippopotamus. ok?"
Yang:"OK."

Then he showed me his satisfaction face. What a kid!