t Flossie could not
very well see his motions in the darkness he said:

"Yes, I heard it. I wonder----"

"Hark!" whispered Flossie. "There it goes again!"

CHAPTER XXI

"HERE COMES SNAP!"

The sound came once more through the darkness to the little Bobbsey
twins, and as they listened to it Flossie and Freddie looked at one
another in surprise. They could just dimly make out the faces of each
other in the dimness.

"Mamma! Mamma!" cried a voice, for it was a voice that had caused the
queer sound; yet it did not sound like the voice of man, woman or child.
"Mamma! Mamma!" it cried.

"Hear it?" asked Flossie again.

"Yep," answered Freddie. "It's a little boy or girl--like us--an' it's
in this cave. I guess lots of childrens get lost here like us. Now I'm
not afraid."

"Mamma! Papa! Mamma!" came the voice again.

"It--it's kind of funny," whispered Flossie to Freddie. "Don't you
think it's kind of funny, Freddie?"

"Yes, but I know what makes it."

"What?"

"It's being in this cave. You know how we used to holler at the hill,
when we went to the country--'member that?"

"Yep," answered Flossie.

"An' how our voices used to come back an' sort of hit us in the face?"
went on her brother.

"Yep."

"Well, that was an echo," said Freddie, "an' that's what makes it sound
so queer here. It's an echo."

"Oh," said Flossie. She had not thought of that.

Once more the voice sounded out of the darkness.

"Mamma! Papa! Mamma!"

"There! Hear it? It's an echo!" cried Freddie.

Flossie listened a moment. Then she said:

"If it was an echo, Freddie, why didn't your voice echo too?"

"Oh,--er--well--'cause I didn't want it to," Freddie made answer. "I can
do it now. Hello! Hello! Hello!" he called as loudly as he could.

And then, to the surprise of the children, back came a voice in answer,
and in more than an answer, for it asked a question. No longer did the
voice call: "Mamma! Papa!"

Instead it cried:

"Hello, there! What's the matter? Who are you and what do you want?
Where are you?"

Flossie and Freddie were so startled that, for a moment, they could only
hold on to each other in the darkness.

Then Freddie found his voice enough to speak. He said:

"Did you hear that echo, Flossie?"

"That wasn't an echo," declared his little sister quickly. "Echoes only
say the same things you say and this--this was different."

"Yes, it was," Freddie agreed. "But maybe it's a different kind of