I'm going to give my niece a pet fish and then take it back.
Why are you thinking of taking it back after giving it?
I want to see her reaction, just out  of curiosity though. No harm done.
How would you feel it someone did that to you? Maybe you don't know how manipulative you're being. Maybe just let her have it.
Fish are dumb anyways. Do you think she will have a traumatic life event over it? I don't think so. I think she'll get over it and then I can give her a puppy to keep instead.
I think you are not really thinking on this. She still might think you will take the puppy from her one day. Not really a cool joke to play on her is all.