The leader says, "This is a dog" to the player on their right.

That player responds, "This is a what?" back to the leader.

The leader replies, "This is a dog."

That player then turns to the player on their right and the exchange repeats, but the first player repeats the question, "This is a what?" back to the leader and the answer, "This is a dog" from the leader back to the second player.

The question and answers therefore go all the way around the circle from the leader to each player, through all the players in between.

Meanwhile, the leader starts "This is a cat" to the left.

VARIATION:
Try the same using these 2 questions and answers:
"This is a hen." "Does she cackle?" "Yes, she cackles"

and

"This is a dog." "Does he bark?" "Yes, he barks."

Try dividing into two teams and seeing who can get through the line first.