Macola:
Ok this is getting seriously weird. Let me just show you, cuz you need to see it to believe it:https://www.youtube.com/watch?v=6h_6V134Ty0
I've got me an air piano!
Now seriously, what the heck!? Why so much induction?
CMOS inputs are usually extremely high impedance (10^10 ohms or greater), so they simply sniff the
surrounding electric fields like a sensitive electroscope (induction is magnetic, that's something else).
In a finished project you should avoid leaving any inputs floating (you can set pins to INPUT_PULLUP).
This is especially important for micro-power circuits as a single floating pin can draw many mA of
current from the supply, whereas a non-floating input pulls millions of times less power.