How do you know an invisible person is near to you and catch him with day-to-day items? [closed]

Some people developed the power of being invisible. They start bothering the population and no scientist seems to know how to develop some advanced technology to detect and stop them.

The government announces: Whoever catches an invisible person without killing him will be rewarded with \$1m. Everybody wants to catch them, so how can they detect and catch them with day-to-day items? Clarification: Day-to-day items are things that everybody can find at home or at a local store and are not expensive. • Sonar and bags of flour! – colmde Apr 19 '16 at 10:06 • I don't think people have sonars in their home. – Pablo Apr 19 '16 at 10:08 • Oh I thought if you were going to earn$1m you might invest in one.... But forgot about "day-to-day items"... – colmde Apr 19 '16 at 10:11
• OK you say, "no scientific seems to know how to develop some advanced technology to detect and stop them" - this seems odd because there is currently technology to detect invisible things. So why can't they (this would affect which day-to-day items would be of use)? The other question is, what are the daily habits of invisible people? Would someone have to hunt for them or could they be often wandering around the streets / my house? – colmde Apr 19 '16 at 10:17
• Let's say that existing technology doesn't detect them. They can be around streets, in shops, your house.. – Pablo Apr 19 '16 at 10:18

Two tactics here:

• First, I'd suggest arming everyone with the Super Soaker type of water pistol and pockets full of bottles of dye.

You can then patrol any area firing frequent short bursts of coloured water. You can fill up at any tap, and you have a side benefit that you will also be able to see footprints on the wet ground.

This method also lends itself to use by anyone, including children, and can be backed up by paintballs once it looks like you have a target cornered.

• Secondly, dogs will still trust their noses, so simply patrol with dogs.
1. Spread flour all over your house. They will make footsteps as they walk.
2. Follow the footsteps to catch them.
3. Using some of your \$1m, buy your other half nice jewellery / a season ticket for their favourite football club to stop them complaining about the flour all over the floor.

I imagine that people would want something that they can easily place down and pick up, so I think a combination of two common materials should do the trick:

Bubble Wrap: You can buy whole rolls of it dirt-cheap, and easily replace it as necessary. Just roll it out on the floor of your home, and you'll be able to easily detect someone attempting to walk around sneakily. Even after the bubbles are popped, the plastic is still irregular enough to crinkle audibly when someone walks on it.

Aluminum Foil: To keep someone from very carefully pulling up the edges of the bubble-wrap, cover the edges of each strip (bubble wrap rolls can easily be multiple feet wide) with a long, unbroken sheet of aluminum foil. If someone tries to pick this up, you'll immediately be alerted to their presence.

Fight invisible with invisible. String up fishing wire across all the doorways in your home, the kind where it's pretty much completely invisible unless it's directly in the light.

Rig up the wire to pull over something loud, say a nearby hidden pile of pots and pans, and to dump a cup of flour/ink/really any opaque substance down over the invisible person. When it goes off, you will know exactly where in the house the invisible person is, and be able to see them because of the flour/ink/whatever covering them.

Then, assuming that they don't really have any other powers besides being invisible, they would probably have the physique of a average human person, so depending on how bad you want that 1mil, just tackle them or something and sit on top of them until the authorities arrive.

This also has the added benefit of not requiring people to spread flour all over their houses and to keep it from settling constantly.