It magically appeared!
This works best if you have a sister who is really young. Like ages 3-7 probably. If your sister is looking for something that you have a lot of (like stuffed animals) take a stuffed animal, or one of whatever she is looking for, and put it at the bottom or top of some stairs, or under the dining room table, etc. Then tell her that you were in the room where you put the object, when you saw a bright flash and all of a sudden that object magically appeared. Watch as they excitedly listen to everything you say. Then, after you are positive that they believe you, tell them the truth to see their reaction.
My 6-year-old sister was looking for hot wheels. When she wasn't looking, I took one that she had already found and placed it at the bottom of the stairs leading to the basement. Then I told her "I just saw a huge bright flash, and all of a sudden a hotwheel appeared at the bottom of the basement stairs! It like, magically appeared! You have to come see it! It just appeared out of nowhere!" So then we both ran down the stairs and into the basement. Her eyes lit up and she got so excited and started telling people (including my dog) Then I said "I'm just kidding, I took that one when you weren't looking" She got so mad lol (