Weird analog read behavior - adjacent pins picking up signal

Hi- I've got a situation where I'm trying to read three analog pins of the Uno, and all three pins are following each other regardless of what the actual signals are doing. For example- if I connect one jumper wire to any one of the pins of the UNO, and leave it floating, all three oscillate wildly. If I ground that wire, all stablize at 0. Same if I peg it with a pull up, all three go high, even though I'm only changing one. I'm using short wires (5cm), and this sort of set up is working fine on another Uno board (I'm trying to recreate a working project on another breadboard so I can do some improvements without taking the original project offline). I expect the oscillation if a pin is floating, but even with all grounded, taking one high takes all high. Pins are adjacent, A1, A2 and A3. Pins A4 and A5 are running as SDA/SCL for RTC and are behaving fine, but I don't know if there is anything shared between the ADC pins. I can post my code if needed, but since this seems to be a physical phenomenon, I thought maybe it wasn't needed?

Thanks!

UPDATE- I was cheating and trying to use 5v and Gnd to represent the output of an LDR with analog read (so I didn't have to mess around with LDRs on my breadboard since I just want to see what happens with my sketch when I cross a threshold). For the sake of troubleshooting, I fully rebuilt the other working project including LDRs as a voltage divider- and now it works as expected. I'm still stumped as to why this would have a different outcome? Only thing I can think of is that I was basically "slamming" the ADC high and low, but should that matter? Maybe ADC doesn't like full 1023 values? Is there capacitive coupling between the pins, which is why they were all copying each other?

An Arduino has only one A/D that is expanded with a muxer (the analogue inputs).
Crosstalk is possible if you don’t drive the pins with an input impedance > 10kohm.
Post a diagram, showing how/what you have connected or intend to connect to the pins.
Leo…

Thanks Leo- I think you nailed it- I updated my previous post because I hadn’t seen anyone reply, but I totally was driving with low impedence. Once I started using actual resistors instead of what I thought was a quick and easy workaround, behavior returned to normal. Thank you for the explanation and education!

The A/D has a tiny sampling/input capacitor that has to be charged/discharged by the voltage source you are measuring.
It takes too long for a high impedance source to charge/discharge that cap when the muxer comes around to sample the pin.
A 100n cap from pin to ground could provide a solid (low impedance) source to sample from.
Another “trick” is to read the analogue input twice, and use the last value.
Leo…

Just so I'm clear, the 10k recommendation is for the following reasons:

10k charges the cap too slowly, so the muxer scans and misses the change. Also 0 ohm (or a few ohm) is too low impedance, and can saturate (not sure that's the right term) the adjacent channels?

soopirV900: Just so I'm clear, the 10k recommendation is for the following reasons:

10k charges the cap too slowly, so the muxer scans and misses the change.

Also 0 ohm (or a few ohm) is too low impedance, and can saturate (not sure that's the right term) the adjacent channels?

Charges and discharges. The voltage on A0 shouldn't change (higher or lower) when the muxer comes past. You could say a low impedance source is "solid" and a high impedance source is "floppy".

No, lower (solid) is better. 0ohm is rock solid. But if the adjacent channel is "floating", it could take the voltage level of the "solid" one next to it. Leo..