If Junior is looking for an interesting demonstration for the first post-COVID science fair, he or she can’t go wrong with this illustration of how human reflexes work.  If you touch a hot stove, your body needs to react before the brain kicks in.  Therefore, you are hardwired to jerk your hand away.

This demonstration illustrates the principle.  A flame is brought close to a simulated human hand, but as soon as it comes into contact with the simulated nerve, the solenoid energizes and jerks the hand away.

The idea appeared 80 years ago this month in the June 1941 issue of Popular Science.  For the young scientist wishing to duplicate the project, all of the parts are readily available on Amazon:  This switch will kick in at 40 degrees Celsius, an uncomfortably warm, but not dangerous temperature.  This 3-volt solenoid will reliably move the hand.  And there’s no need for the old-school dry cell batteries shown above.  Alkaline D cells will work very well, especially if you have a battery holder for them.   And while Junior can make the hand out of cardboard, he or she will be virtually guaranteed the blue ribbon with a realistic looking plastic hand.

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