Non-mechanical timing device for 10–40 seconds without electricity

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I need to design and create some sort of device to measure an interval of time between 10 and 40 seconds. I can't use any electrical or mechanical things. I had ideas of puting water in a funnel and letting it fill up a cup in which I've measured how much water equals a second for example. I also thought of dropping marbles off of a low-incline ramp and timing it's drop to the base. The catch is my physics teacher is making everyone in the class do a different project, the water measuring one has been taken and so have the marbles.
Does anybody have any ideas on what I can do? I have no problem designing, building and testing the device I just need to get going here.

Thanks All
 

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Given that both the water and marble clocks are "mechanical", I am at a total loss. What exactly is ruled out by that term?
 
I used the wrong term. We can't use anything electronic appliances such as fans, dremel tools etc.
 
A-ha. In that case, how about a pendulum? Has that idea been taken?
 
well i was considering using a pendulum, but the period is bound to change during a duration of up to 40 seconds - I am allowed only a 2 second grace period
 
It's still a possibility to do something involving marbles. One other person is already doing something where they mark on a track every second, and as the marble falls they count how many seconds the marble passes through. A good idea, but unfortunately I can't do the same thing
 
Sand timer? If sand's taken, how about a sugar timer? Or a salt timer?
 
brewnog said:
Sand timer? If sand's taken, how about a sugar timer? Or a salt timer?

I am always one for cutting corners, but yup somebody is connecting two 2-liter soda bottles and filling it with sand. his friend tried to do sugar and it's not allowed
 
Why do you dismiss pendulums so easily?

You've got other versions than the grandfather clock variety, you know..
Look up, for example, torsional pendulums to get another pendulum type
 
arildno said:
Why do you dismiss pendulums so easily?

You've got other versions than the grandfather clock variety, you know..
Look up, for example, torsional pendulums to get another pendulum type

a torsional pendulum might work, I'm researching them as we speak
 
Also, there are other types of harmonic motion: like a mass and spring.
 
I came up with another idea that hadn't been taken - it's hanging a slinky from a hanger with a mass on the bottom and letting it stretch vertically, something like in the diagram seen here

link

do you think that this could be semi-accurate?