Electronic – Obtaining a triangle wave from a square wave using a LM324N op-amp

Try holding the wire in an insulating holder - say on the end of a toothpick or wooden chopstick etc. Results are likely to be different.

You should provide a circuit diagram of EXACTLY what you are doing BUT if pin 6 is "floating" normally then your body will pick up mains hum capacitively from the surroundings and couple it into the pin. ALL input pins must ALWAYS have defined conditions if you want defined operation to happen. If you don't want defined operation then you can connect them any way you wish :-).

The voltage on pin 2 is pulled up by the base current from an internal PNP transistor, maybe 500nA. So when the output of timer 1 is high, the other side of the capacitor is also high. When the output of timer 1 goes low, the voltage across the capacitor (call it zero for the sake of discussion) does not change instantly (that's what capacitors do) and thus the trigger input is pulled low and the second timer starts.

Incidentally, that's not very good practice, though it probably works, there should be a resistor of 20K or something like that from the trigger input to the supply. It might even sort-of work with a CMOS 555, depending on the frequency vs. leakage currents, but a resistor will make it work for sure.

Internal schematic from here:

The capacitor charges from the base current at pin 2 (Q10). When the output of timer 1 goes high again, the capacitor is discharged again.. I think initially by two diode drops above the supply voltage through the well used to isolate the lateral PNP, then it drops a bit to supply the bias current. Note that this usage breaks the absolute maximum input voltage spec on the trigger input, but hey, lots of folks do it.