The Professor narrows his eyes as he sees the fireball. "That's a hologram," he says.

"Put your hand near it, Professor. You can feel the heat."

He frowns, but he brings his hand closer. "...hmph."

"It's a real fireball," I assure him.

The Professor reaches into his desk drawer and pulls out a cigarette.

I smile.

The Professor brings his cigarette closer and closer to the fireball, without taking his eyes off mine. As for myself, I am keeping my eyes on the cigarette end; at the moment when it pierces the fireball, it lights, and a wisp of smoke escapes.

The Professor looks at the burning cigarette end, then look over at me. "Congratulations," he says. "Now do you want to tell me how you faked it?"

"What makes you think it's fake?" I ask.

"Three reasons," says the Professor, taking a pull on his cigarette. "Firstly, today is April the first. Secondly, you have a well-deserved reputation for pranks; thirdly, I know for a fact that your thesis is on personal-wearable hologram generators, not fireballs."

...I sigh. "Yeah," I admit. "You got me."

"So how did you do it?" asks the Professor.

"The generator produces light," I point out. "No-one says it needs to be limited to visible range. This particular little fireball puts out infrared light as well, which makes it feel hot."

"And the cigarette lighting?"

"I have an infra-red laser remote lighter up mt sleeve. Honestly, the worst part was aiming it."

The Professor nods. "Well done," he says again. "A very convincing combo."

"Thank you, Professor."

And thank you for being willing to help put into widespread adoption the kind of technology which would mean that no-one will notice what my sister is doing if she starts throwing her own fireballs about...