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Electric Kettle
Age 14 to 16
Challenge Level 
The equation of the line is $T = 5R +20$, so your gradient should be close to 5.
To heat water to 100 C you need $R = \frac{100 - 20}{5} = 16$ ohms. If the resistance is zero then $T = 20$C. In practice it is really hard to have a circuit with zero resistance. You need special conditions such as a low temperature.
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