This is a simple instructable mainly focused on the principles of Physics and will teach the following in an easy to follow tutorial format:

Dynamics, equilibrium, fulcrums, leverage, counterweight and the principle of moments(and possibly how to use a calculator)

Supplies

1) a long plank of wood(at least 1 meter long and thick enough to hold about your body weight+the weight of 4 7L bottles)

2) a rotating stool(the type you sit on!)

3) 2 7L bottles

4) securing straps that can hold a lot of weight

5) a calculator(not strictly necessary but useful)

6) a helpful(and hopefully strong) adult

Step 1: "horrible" Maths and a Teeny Bit of Theory

We need to calculate only two things(this is where the calculator and the helpful adult come in):

the length of wood needed on one side to counterweight the 7L bottle
the willingness of the adult to help(any value over 5 but under 7 is good, perceive this yourself when you ask them to help you with an equation)

let's start with the length of wood, for which we. need a chomper of a formula, Fe de = Fl dl which simplified(yes, even I had to look such a formula up) means that one weight multiplied by the length from the fulcrum(the point of balance) is equal to the other side's length multiplied to its weight on a balanced seesaw

so if the mass of the 7l bottle is equal to 7KG, then we need to find the ratio so we need to do:

your weight÷7=x (you can round to a realistic value of 1 significant figure)where the ratio is 1:x and multiply by the length of your plank divided by x+1(hard, right?)

as an example, 70÷7=10 so the ratio is 1:10 so if the plank is 100 cm long, we multiply both sides by 100÷11 which is about 9 so it is 9:90, which is close enough to the full length(it is 1 off), although you might have to experiment a bit later on, it is worth not having to deal with numbers like 9.090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909090909 reoccurring forever(this is 400 numbers after the decimal point)

Step 2: Some Hands on Work

try out the equation by using the helpful adult's help to set up the experiment by measuring and holding the plank flat onto the stool and securing it as in the figures, you will find that 1 and 3 balance, while 2 doesn't, as the equation is not equal(you might want to change the third one to 10KG and 1KG!)

Step 3: Finalising Your Work

this stage is simple...

oremlay ipsumyay olorday itsay ametyay , onsecteturcay adipiscingyay elityay , edsay oday eiusmodyay emportay incididuntyay utyay aborelay etyay oloreday agnamay aliquayay . utyay enimyay adyay inimmay eniamvay , uisqay ostrudnay exercitationyay ullamcoyay aborislay isinay utyay aliquipyay exyay eayay ommodocay onsequatcay . uisday auteyay irureyay olorday inyay eprehenderitray inyay oluptatevay elitvay esseyay illumcay oloreday euyay ugiatfay ullanay ariaturpay . excepteuryay intsay occaecatyay upidatatcay onnay oidentpray , untsay inyay ulpacay uiqay officiayay eseruntday ollitmay animyay idyay estyay aborumlay . isay ochay anslatumtray adyay anglorumyay , utay ingeniumyay , edsay untsay olumsay ottay adusgray utyay osvay otestpay ascendereyay etyay ochay estyay enusgay unfay allengechay opray etay , utyay instaryay iccosay uamqay uriosuscay esyay , etyay ioscay opray ertocay , uodqay egoyay onatuscay adyay anslatetray isthay exttay , osvay ustuloyay adyay octavumyay inquisitivenessyay isay onnay aculosamay occultusyay untiusnay , edsay etiamyay ayay ecemday isay idistivay eamyay inyay extutay egoyay umsay ursorcay exyay erbisvay , itayay uncnay alevay

and enjoy, genius