Warning: It is actually a simple "puzzle" which is silly but debatably unique as it really happens to me in real life.

I'm amazed with nowadays technology. When I go to one of the shopping mall in Singapore, I found this: a public charging station with fingerprint lock system! (Well, at least I'm amazed because I couldn't find something like this in my origin country, lol.)

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On this machine, there are 10 slots (labeled 1 to 10 from up to down) for charging and a single monitor screen. One slot is strictly restricted to be used by only one device. The slot color light denotes if the slot is used or not: red means used and green means unused.

The procedure of the machine is like this (for simplicity of the puzzle):

  • There are 2 options on the screen: "Charge" or "Collect". "Charge" only available to be chosen if not all slots are red, likewise "Collect" only available if not all slots are green.
  • If user choose "Charge":
    • The user is asked to tap his/her finger on fingerprint device 4 times.
    • If all of those 4 fingerprints are similar, the first fingerprint will try to be saved in its database. If all of those are not similar, the screen will show error "Different fingerprints detected!" then go back to the beginning of procedure.
    • If the fingerprint is not in current database, the fingerprint is then saved and the uppermost green slot will be open. But, if it is already in current database, the screen will show error "Only 1 device/person is allowed!" then go back to the beginning of procedure.
    • After the slot is opened, the user is supposedly to charge his/her device inside the slot.
    • The user then close the slot, the slot's color will change to red. It then goes to the beginning of the procedure.
  • If user choose "Collect":
    • The user is asked to tap his/her finger on fingerprint once.
    • If it matches with any fingerprint on the database, then the respective slot will open. The user supposedly take his/her device out. After the user close the slot, the slot's color will change to green and the fingerprint is removed from the database. It then goes to the beginning of the procedure.
    • If it doesn't match with any fingerprint on the database, the screen will simply show error "Fingerprint is not matched!" then go back to the beginning of procedure.

I and 2 of my friends (say B and C) want to charge our devices. The machine shows all of 10 slots are green. B chooses "Charge" and taps his finger, then slot 1 is opened. Unluckily, the charge cable on this slot doesn't work! B then closes the slot, it turned red. We don't know how the logic works before, so B tries to collect then charge again (with his finger), willingly another slot is opened. But of course, slot 1 is opened again.

Now this is the fun part. B closes the slot, it turned red. C then chooses "Charge" on the screen and tap his finger. The slot 2 is opened, but, C asks B to charge B's device on this slot. C closes the slot then asks me to choose "Charge" and tap my finger. The slot 3 is opened, then C puts his device on this slot and closes the slot.

At the current state, slots 1 to 3 is red and my device is still on my pocket. Slot 1 uses B's fingerprint, slot 2 uses C's fingerprint, and slot 3 uses my fingerprint. But somehow, we then successfully manage to charge all of our devices. How?

  • 2
    $\begingroup$ You know you have ten eligible fingers each right... $\endgroup$ Jan 19, 2018 at 2:32
  • 1
    $\begingroup$ @theonetruepath Not necessarily; the puzzle is tagged "lateral-thinking". For all we know, A, B, and C might be single-fingered aliens. ;) $\endgroup$ Jan 19, 2018 at 2:35
  • 1
    $\begingroup$ Is this too much for lateral thinking. $\endgroup$ Jan 19, 2018 at 3:06
  • $\begingroup$ As a Singaporean, I have to confess that I have never seen any of these things before. I also find it hard to imagine the average Singaporean willing to part from their smartphones for long enough to use these. $\endgroup$
    – Xenocacia
    Jan 19, 2018 at 8:44

2 Answers 2


You all

use your other fingers to unlock the other slots :|

Those devices sound like they take one finger to open a box, so you can easily open the other slots by using other fingers

Or maybe

You didn't say you charge them all at the same time. Just take turns using slots 2 and 3 :)

  • $\begingroup$ but.. but.. single-fingered aliens! ;D $\endgroup$ Jan 19, 2018 at 2:40
  • $\begingroup$ Just.. just one finger total? not even one in each arm? not even toes? gosh, poor mono-appendage creatures :'( $\endgroup$
    – votbear
    Jan 19, 2018 at 2:44
  • 2
    $\begingroup$ The real puzzle is how the single-fingered aliens manage to plug charging cables into their phones. $\endgroup$ Jan 19, 2018 at 3:02
  • 1
    $\begingroup$ Yeah, i was about to comment on how inconvenient it must be to operate a phone when you only have one finger... but then i realized that the question never did mention anything about phones. Just 'devices' ;) $\endgroup$
    – votbear
    Jan 19, 2018 at 3:32
  • $\begingroup$ Yes, the correct answer is, indeed, using other finger... Funny when the screen shows "Only 1 device/person is allowed!" error meanwhile we can use up to 10 (or 20?) slots. Welp. $\endgroup$
    – athin
    Jan 19, 2018 at 3:38

B opens the broken slot 1 again as a 'trap' for an unsuspecting 4th person/1-fingered alien.


They will be assigned the broken slot #1 and after figuring out the charger is broken, you can - even without lying - tell them you tried as well, but unfortunately it's broken and hope they won't bother to once again unlock their slot. (Why would they?)


This allows slot #4 to be used and also all other 6 slots can now be used and reused over and over again throughout the day as long as the poor holder of #1 does not return or the machine is somehow reset/emptied by staff.


The question remains whether it's ethical to let 1 phonebattery die to save so many others.

  • $\begingroup$ Personally, I would simply have left slot one locked without a device in it using my left thumb and moved on to the next slot. $\endgroup$ Jan 20, 2018 at 11:37

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