A man enters a room and presses a button. Within seconds he loses 20 lbs. How did he lose the weight?

  • 7
    "Lateral Thinking" Oh! I get it! – Vnge Sep 24 at 18:07
  • 8
    Nitpick: He loses 20 lbf. His mass stays the same. – ohno Sep 25 at 8:52
  • 6
    He’s carrying 20 pounds of groceries in a bag, which he coincidentally drops on the floor a few seconds after pressing a completely unrelated button. – Janus Bahs Jacquet Sep 25 at 13:58
  • @ohno lb is a unit of weight (as well as unit of mass), and the question says he loses weight. – armb Sep 25 at 16:30
  • 2
    The pound is also a unit of currency. – Acccumulation Sep 26 at 15:51

17 Answers 17

up vote 55 down vote accepted

He entered

an elevator and pressed the down button. The downward acceleration from the elevator reduces his apparent weight.

  • 2
    Doing the math for the average weight of 136lbs(62kg), acceleration of the elevator must be about -1.5 m/s^2. To put that in context, the average acceleration of an elevator is around -0.64 m/s^2. – David Starkey Sep 24 at 20:42
  • 5
    lbs is mass, not weight... but blame it on the question. – vsz Sep 25 at 6:10
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    @vsz: Nope. lbs is weight, not mass. You have it backward. "slugs" are the unit of mass in the Imperial system. – Beska Sep 25 at 13:02
  • 1
    The pound is a unit of both mass and force. The poundal is unambiguously a unit of force, which will give a mass of one pound an acceleration of one foot per second per second. The slug is unambiguously a unit of mass, which will be accelerated at one foot per second per second by a one pound force. – armb Sep 25 at 16:32
  • 3
    @Beska : No, lbs, as currently legally defined, is mass. It's lbf which is force. – vsz Sep 25 at 16:38

Since it's tagged 'lateral-thinking', an alternative that blatantly ignores the question's title is that:

The man is in the UK, and he just pressed 'OK' button to confirm a £20 purchase

  • 1
    I thought of this first, but it specifies "weight" in the question, so this answer is invalid – Quintec Sep 24 at 18:17
  • 7
    :jedi-hand-wave: Let's just call that a 'Financial Burden' then :P – Dave B Sep 24 at 18:44
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    "Pounds" is abbreviated as "lbs.," which is never used for the currency. – Sneftel Sep 24 at 19:34
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    However, did you know that the "£" symbol and the "lbs." abbreviation both have the same origin? They both essentially mean Libras (scales, used for measuring weight). – peaceoutside Sep 24 at 20:30
  • 2
    Isn't this less "Lateral thinking" and more "Vertical thinking" ? – Selkie Sep 25 at 16:56

He presses the button to start a machine. It catastrophically fails within seconds and takes off the lower part of his leg, which weighed 20 lbs.

  • 20
    Should I be concerned this was my first thought too? – Robert S. Sep 24 at 17:35
  • 1
    Somewhat along those lines would be someone in one of the Saw movies going to the next room and, well... lets just say traps and stuff. – user7393973 Sep 25 at 8:35
  • It is a little too much deviation from the story – Quark-epoch Sep 28 at 9:49

For an answer which exactly fits the question and the OP's clarifications...

He is wearing a diver's weight belt with a quick-release catch.

He is:

Holding a 20 lbs of iron. The button activates a very powerful magnet.

Or:

In a room with some sort of anti-gravity (or, rather, gravity manipulating) feature. Just something that would slightly adjust the gravitational constant to make you lighter.

  • no he loses 20 lb body weight – Quark-epoch Sep 24 at 16:50
  • 7
    Probably should specify that in the question :-) – Greg Sep 24 at 16:52
  • well, rot13(va nagv-tenivgl ur jbhyq ybfr nyy uvf obql jrvtug abg whfg 20 yof naq 20 yof pna'g or uvf obql jrvtug) – Quark-epoch Sep 24 at 16:54
  • what is he holding? – Quark-epoch Sep 24 at 16:55
  • 1
    He had a metal leg? – Sulthan Sep 25 at 11:36

A somewhat less morbid variant of an answer that's been posted a few times:

He has a prosthetic leg that weighs 20 pounds, and the button releases its clasp.

  • that is dammed clever. good one! – Fattie Sep 26 at 20:24

The man previously lost 20lbs and without knowing this he came to check his weight on a weight machine and pressed button to check his weight apparently in seconds the result popped up showing he lost 20lbs, looks very natural and correct answer ! dont ask me exactly how he lost 20 lbs :-)

I know this already has a correct answer, but I'd like to offer an alternative:

The man works as a stocker and is holding 20lbs worth of cardboard. He enters the back room where he puts the cardboard in a baler and crushes it with the push of a button.

  • 1
    According to my calculations, 20 lbs of cardboard is 152 square meters of cardboard, approximately a 12 by 12 meter piece of cardboard. No matter how you fold it, that'll be impossible to hold by one man. – Jacco Amersfoort Sep 27 at 8:11
  • He loses 20 lbs BODY WEIGHT – Quark-epoch Sep 28 at 9:49

He walked into a Star Trek transporter (or similar) and transported himself to a planet where gravity was less strong.

The button...

... triggers a guillotine which chops a leg off (apparently ~20lb for an adult male)

It was an air lock of a submarine and filled the chamber with water, the buoyancy reduces his weight by 20 lbs.

or

The button published a new law that vital organs are owned by the government rather than the individual. He is no longer the legal owner of that weight.

or

The man was head of the International Committee for Weights and Measures and the button was publishing a new heavier definition of the pound.

Simple, the man pressed

the buzzer on his door and let in a cleptomaniac bodybuilder who proceeded to steal his 20lb dumbbell. He lost his weight because he's too trusting of other people.

Upon pressing the button,

The entire room starts to accelerate downwards (for example, he is inside the lift) at an acceleration of a

So

Given that his original weight is Mg - Ma = 20lbs, we calculate the a given his mass M.

  • 1
    Nice answer, but next time you should really put your answers in spoilers :D – Kevin L Sep 26 at 2:22

My first thought was:

He entered an elevator and pushed a button, causing the elevator to go down. The acceleration downwards is credited with making the man 20 pounds lighter, if only for a brief moment.

Then I thought of this:

If "lbs" stands for British pounds, then the man could have been at a gambling casino. He could have bet twenty pounds on red in a game of electronic roulette. When he pushed a button, the roulette ball landed on black, causing the man to lose his 20 pounds.

Thinking about it some more, I thought:

If the twenty pounds are truly weight, then the man could have simply been separated from something on his person. This could be an arm or a leg (separated by, oh, let's say a button-activated-spring-loaded-sawblade), or just something he was carrying. Maybe he was a delivery man delivering a 20 lb. package that was ordered on-line. He rings the doorbell, then within seconds a spring-loaded-saw... I mean, a grateful customer takes the 20 lb. package from the delivery man.

But then I thought some more, and I came up with:

Maybe he bought a collection of Richard Simmons' "Sweatin' to the Oldies" videos (I've seen some for around that price). I can't really explain how he'd lose so much weight "within seconds," unless I reinterpret "seconds" to mean "going back for seconds" (often heard when eating a meal). Maybe he liked the videos so much, he exercised a second time, or managed to lose enough weight despite having more portions of food.

  • I like the thinking. The first part is already said but the second and fourth parts are intriguing – Quark-epoch Sep 28 at 9:46

The button was connected to explosives hidden in this, this, this and seventeen more such buildings. When he presses the button, the twenty LBS are gone.

He entered a vertical wind tunnel which got up to speed slowly as he stood there after pressing the button

He enters lift, presses the button downwards. Psuedo acceleration upwards works here against gravity and weight measured is less now.

Let M be his mass and mg be weight before (outside lift)
Losing 20 lbs-
Pseudo acceleration is (Mg-20)/M unit length per square unit time

  • 1
    Exact same solution was already posted by Quintec. In addition please not to reveal spoilers by using >! at the beginning of a paragraph. – mpasko256 Sep 25 at 10:05
  • I didn't see the response. Just the question. And I also gave the equations, so it's not the exact same. – User12345 Sep 25 at 13:48

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