Why some animals have tail but not for the other ?

Recently, Sai Sai learned about the life cycle of frog at Kentville school.
This morning, he kept jumping in the living room.

"What are you doing ?" Mama asked.
"I'm a bunny !" Sai Sai replied.
"oh... but you look more like a frog ..." Mama challenged him.
"really ? ..." Sai Sai was a bit disappointed.

Then, he taught me the life cycle of frog ... from egg to tadpole to baby frog ...

"Tadpole has a tail, but when it becomes a frog, there's no tail" Sai Sai said.
"Why ?" Mama asked.

"If there's a tail, the frog will step on it when it jumps."
"oh... how about dog ?"
"It's okay, as the tail of a dog is very short."
"hm.. how about giraffe ?"
"No problem, as giraffe is very tall."
"and elephant ?"
"The same, elephant is very tall and the tail is not that long."
"then, how about lion ? lion is not that tall but has long tail .."
"but lion is very careful, it won't step on its tail !"
"......."

ha..ha...ha...