# What does it take to solve a hard one?

By measuring once you shall know me,

By measuring twice you shall not.

Though I'm needed for action and getting things done,

The more I speed up, the slower you'll run.

When I come to a stop, it's the end, all is done!

• I know on other SE Websites you're not supposed to post the answer in the question, is it the same for this website? – André Jul 25 '18 at 13:22
• If you'd like to give reasoning behind your puzzle, please make it an answer - meta link. There's nothing wrong with making your own "OP wrap-up". For more info on making one, see this meta – David Starkey Jul 25 '18 at 14:06

Are you

Time?

By measuring once you shall know me,

By measuring twice you shall not.

You can measure the time, but once you measure again, it is different.

Though I'm needed for action and getting things done,

You need time to get stuff done.

The more I speed up, the slower you'll run.

Could be a reference to time dilation, or the stereotypical slowing of actions when traveling fast during SciFi...

When I come to a stop, it's the end, all is done!

When time stops, the end has come!

Title:

It takes time to solve a hard riddle

• I'm not sure whether @Jesse or I finished first, but I like his answer a little better... – elarr Jul 24 '18 at 13:47
• Ha ha ha, exactly what I thought about your answer! +1 – hat Jul 24 '18 at 13:49
• Given the title I agree Time works better. – Joe Jul 24 '18 at 16:33
• About measuring once or twice: "A man with a watch knows what time it is. A man with two watches is never sure." - Segal's Law – B540Glenn Jul 24 '18 at 19:02
• I think this is it, but "The more I speed up, the slower you'll run." likely refers to the perception that time moves faster as one gets older. – Wazoople Jul 24 '18 at 20:50

Could it be

A person's age

Why:

By measuring once you shall know me,
By measuring twice you shall not.


A person's age changes constantly

Though I'm needed for action and getting things done,


A person develops skills with age, starting from birth

The more I speed up, the slower you'll run.


Older people can't run as fast as they used to.

When I come to a stop, it's the end, all is done!


You stop ageing when your life is over

New here, but I really like this one (including the other answers), but it immediately made me think it was:

Breath

By measuring once you shall know me, By measuring twice you shall not.

"A measured breath" refers to breathing in for a set period (seconds or counts usually). On the breath in, the air is in your body, so the exhale, it would be out of your body and gone.

Though I'm needed for action and getting things done, The more I speed up, the slower you'll run.

Your breathing speeds up as a result of getting more and more tired, not the opposite, but that seems largely not relevant to most riddles?

When I come to a stop, it's the end, all is done!

Pretty self-explanatory here. If you stop breathing, it's over. The converse of this is also true.

Could you be

a heart

By measuring once you shall know me,

Can measure heart rate

By measuring twice you shall not.

Heart rates can fluctuate since the initial reading.

Though I'm needed for action and getting things done,

Your heart pumps blood, which helps power the rest of your body..

The more I speed up, the slower you'll run.

Having a fast heart rate will could indicate you're tired and make you run slower

When I come to a stop, it's the end, all is done!

Sounds like you have come across

Errors

By measuring once you shall know me,

Errors are common when enough planning is not done beforehand. A common saying in wood working is, "Measure twice, cut once" as to avoid making errors.

By measuring twice you shall not.

Ties neatly to the above statement as being more prepared makes errors less frequent.

Though I'm needed for action and getting things done,

Errors are part of the process in anything. Very rarely do things always go 100% to plan, but working through them is necessary to get things done.

The more I speed up, the slower you'll run.

The more errors you encounter while working on something, the longer it will take for you to finish your task while you correct those problems.

• I guess if we run out of errors a lot of people are out of jobs! – Michal Ciechan Jul 24 '18 at 17:15

You are

Heat

By measuring once you shall know me

You can measure temperature

By measuring twice you shall not.

...but it's an entropic measurement, so you won't get the same reading twice? The act of temperature measurement can have an affect on the measurement itself by adding or removing energy from the system, so measuring temperature twice will never give exactly the same result

Though I'm needed for action and getting things done

Combustion (and many other) engines run on heat differentials

The more I speed up, the slower you'll run.

Heat exhaustion causing runners to slow down

When I come to a stop, it's the end, all is done!

The inevitable heat death of the universe

• A good first answer and explanation :) Make sure to take the tour to earn a badge! – Sensoray Jul 25 '18 at 15:31
• I don't like all of the "measuring twice" explanations of "it changed", but I like this one because the act of measurement causes the change. – Sam Liddicott Jul 25 '18 at 15:56
• @SamLiddicott That was the only one I was iffy on. Your addition to it makes more sense. – atwig Jul 25 '18 at 16:01

By measuring once you shall know me, by measuring twice you shall not.

You can only measure the position or speed of an electron at any given time. To have both at once would mean neither are correct.

Though I'm needed for action and getting things done, the more I speed up, the slower you'll run.

Energy is needed to do work. Any work done will encounter friction and produce heat. The closer you are to heat stroke, the slower you'll run.

When I come to a stop, it's the end, all is done!

• For the title: CLR (Calcium Lime and Rust remover) – Mazura Jul 24 '18 at 17:32

Are you

bitcoin ?

By measuring once you shall know me,

you see its value

By measuring twice you shall not.

it changes that fast that makes you think that you don't know it

Though I'm needed for action and getting things done,

bitcoin is needed for transactions and you finish a deal by paying with it

The more I speed up, the slower you'll run.

the faster it changed ,the more you will think about investing in it

When I come to a stop, it's the end, all is done!

when it loses a lot of value all investors are gone

time

By measuring once you shall know me,

By measuring twice you shall not.

This is a reference to the well known saying attributed to Lee Segall: "a man with one watch knows what time it is, and a man with two watches could never be sure" https://www.barrypopik.com/index.php/new_york_city/entry/a_man_with_one_watch_knows_what_time_it_is_a_man_with_two_is_never_sure

Though I'm needed for action and getting things done,

I can't do it if I don't have any/enough time

The more I speed up, the slower you'll run.

If time slows down, we can do more, if it whizzes by, we get little done. Is a faster rate of time is an increased supply of time? It certainly doesn't help!

When I come to a stop, it's the end, all is done!

If time stops, nothing happens (a theme in some science fiction), but if we personally run out of time, then we die.