The Periscope Spy

It is your best friend's birthday, and the whole class is in on the secret. The lights are low. The cake is hidden behind the teacher's desk. Everyone is crouched down, ready to leap up and shout SURPRISE the instant your friend walks through the classroom door. Someone is walking them down the corridor right now. And the timing has to be perfect. Jump up too early and there's nobody there yet. Too late and your friend has already peeked into a dark, quiet room, decided it's boring, and turned back — surprise ruined. So your team needs a lookout at the door, watching the corner of the corridor for the exact moment your friend appears. Here is the trouble. To watch the corridor, the lookout has to lean their head out past the doorframe. And the precise second that head pokes out — your friend spots it. One glimpse of a lookout peeping around the door, and the whole surprise is blown. You need to watch around the corner while your head stays hidden inside the room. Your eyes can't do that. Eyes only look straight ahead — they can't bend around a doorframe. But maybe the view can be bent, even if your eyes can't. Today, your team is going to build the thing that does exactly that — a spy tube that looks around corners for you.