strange arduino oscilloscope effect

today I made an arduino oscilloscope. it is a simple analog read and serial plotter. without connecting the probe to anything on the serial plotter appeared a sinusoidal waveform at about 0.2 Hz.

then I tried to connect the probe to my hand:the amplitude increased and the waveform became trapezoidal.

the strange thing is the very low frequency.

Hello there!

This sounds like a very interesting project. Can you post your code using the code tags as well as any circuitry so we can see what you are doing?

without connecting the probe to anything on the serial plotter appeared a sinusoidal waveform at about 0.2 Hz.

Floating pins will take up about any electrical noise available in your environment. The same is probably try for your body as the body voltage is probably to low for the Arduino to detect, so it's behaving similar as if nothing is connected.

at about 0.2 Hz.

How did you estimate this frequency?