I do this a lot to my siblings. It annoys the way too much but luckily they are on the shower, meaning they can't hurt me until later on.

What you need
• someone in the shower
• light on the outside of the room.

What to do
When your person is in the shower stand by the switch (which must be on the outside or it'll be pretty awkward).

Turn off the switch and wait. Wait until you hear them shout. Turn the light back on but repeat afterwards.

You can do it for as long as you want but once you hear the shower turn off, run as fast as you can into a hiding spot before they can reach you and inflict any harm.

(I would also not do this when my parents are around as they will yell at you causing you to stop.)

Good luck.