build a battery .....
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Have a look at this website: https://en.wikipedia.org/wiki/Lemon_battery
Key variables: the two metals should be far apart in the electrochemical series. E.g. Magnesium or zinc for the negative electrode; copper, silver (or platinum if you can afford it) for the positive electrode.
As the website correctly states "a typical voltage is 0.9 V with lemons. Currents are more variable, but range up to about 1 mA (the larger the electrode surfaces, the bigger the current)"
A small red LED needs about 1.6 to 2 volts ( See: https://www.pinterest.ie/pin/372109987940720425/?lp=true ) and 5 to 20 millamps. Simply measuring the open voltage (i.e with no current being drained) will not tell you if a chemical cell will drive an LED. LEDs of other colours (orange, green, blue) require successively higher voltages.
As the diagram on the wikipedia site (above) shows, you should arrange two or three cells in series. Use electrodes with large surface areas in contact with a conducting solution.