M: Explain to me the physics that make it so that when I try to water a plant out of a glass, it always ends up dribbling a little water down the side. But when I pour it out of something shaped like a thermos, generally it doesn't do the same thing. Then give me a little guidance into what will or will not have that happen, because that was just an example I gave, but it happens quite often, particularly in cooking situations, and I need to figure out the physics of it well enough to stick in my own head so that I can be prepared. It's one of those things that has gone on for years, but I just never have had the time inclination and situation reminding me that I needed to do it up until now.

G: That dribble is caused by surface tension and wetting—the same family of effects that make raindrops bead up or water “cling” to a spoon. Here’s how it works:

1. Why it Happens (The Physics)

When you pour water out of a glass:

Adhesion > Gravity (at first):
Water molecules are attracted to the glass surface. If the rim is not sharp, that adhesive force can be stronger than the pull of gravity on the moving stream.
Cohesion + Surface Tension:
Water molecules stick to each other as well as to the surface, forming a thin film that tries to “stay connected.” So when you tip the glass slowly, the stream follows the curved rim and crawls down the side.
Laminar Edge Path:
The rounded lip gives the liquid an easy path to cling to. Once it starts, the adhesion–cohesion loop keeps pulling more water along the surface.

When you pour from a thermos or bottle, the lip is usually thin, angled, or outward-flared, which:

Breaks the contact cleanly (less adhesion area).
Allows air to slip between the container and the falling stream.
Reduces the “bridge” of water that can cling and crawl back.

2. Geometry and Surface Clues

You’ll get dribble if:

The lip curves inward (most drinking glasses).
The material is hydrophilic (glass, ceramic).
You pour slowly or at a shallow angle.
The edge is thick or blunt, giving water more surface to adhere to.

You’ll avoid dribble if:

The edge is sharp, thin, or has an outward spout.
The surface is hydrophobic (coated metal, Teflon, some plastics).
You pour decisively—a faster stream breaks tension sooner.
The rim design includes an air gap (like kettles and carafes).

3. Practical Rule of Thumb

Rounded rim → dribble risk.
Outward flare or pour lip → clean break.
Slow pour → higher adhesion time.
Faster pour or sharper edge → clean stream.

If you’re stuck with a dribbly vessel (like a bowl or mason jar):

Touch the rim briefly with a wooden spoon handle or chopstick—the stream will follow the new path cleanly down the stick, bypassing the edge.
Or pour along a thin edge (like a butter knife) resting against the lip—same principle.

4. Sticky Mental Shortcut

Think of it like this:

Water follows what it touches until the drop breaks free.

Sharp edges, nonstick materials, and faster movement help that break happen cleanly. Rounded lips and slow pours let adhesion win.

M: Yeah, that's kind of what I thought, although I hadn't thought about the material involved. I don't think I was aware of the rim width having a part to play in it. I don't have a watering can, I don't think. Otherwise I would use that. Although I'm not sure if I would want to, because generally what happens is I remember to water the plants when I go to get some water myself in the morning. It doesn't seem like much, but the extra step of finding and making sure that the watering can is ready to use is enough to make or break a chore that you currently can do in passing, even if that chore sometimes leads to the follow-up chore of cleaning up the spilled water.

G: That makes complete sense. What you’re describing is the tradeoff between habitual friction and cleanup friction. The tiny “activation cost” of going to find the watering can is often higher than the occasional spill, because the former interrupts the routine and the latter just adds a few seconds after. The brain prefers the path that keeps momentum.

If you want to minimize spills without changing your pattern:

Pick a glass with a thin, sharp rim — even a cheap wine glass or measuring cup will pour cleaner than a thick tumbler.
Angle farther and pour faster — the quicker you establish a steady stream, the less adhesion time the water gets.
Rest the edge on the pot’s soil or inner rim — that gives the stream a short drop, which breaks tension and avoids the “crawl.”
Optional low-friction fix: wipe a trace of cooking oil or wax around one small section of a frequently used glass’s rim. That one spot becomes a consistent clean pour point.

That way you keep the one-step convenience, just with physics on your side.

M: I'll probably have to find which of our drinking glasses works best according to those. The problem is, again, I get a drink and then pour some for my plants from the same glass. And I have a father who, in the morning, obsessively cleans up the kitchen before he eats. I mean that, literally, it's an either-nature-or-nurture tick he's got where he can't stop himself. So if I try and leave a glass out for water, there's a 50-50 chance that he'll feel the need to wash it, like put it in the dishwasher. So even if I find one that works really well, there's a good chance that 50% of the time I'll have to use the second-best one or something like that. It is kind of funny that the topics came up, because what you're talking about with the habitual friction is similar to the physical surface tension. It doesn't seem like much, but it can lead to an entire alteration of how things go, whether you're talking about splashing out of a cup or splashing out of the planned routine of the day.

G: That’s a very sharp parallel — and it fits perfectly. Surface tension and habit tension both resist small breaks in continuity. Once momentum is interrupted, whether by a droplet clinging to glass or a parent’s compulsion to tidy, energy is lost to friction that shouldn’t even be there.

You’re basically describing two linked systems of path dependency: one physical, one behavioral. The physics version is adhesion versus gravity; the habit version is convenience versus disruption. Both reward a clear, low-friction channel.

Practically speaking, your best move is probably to standardize a “pour glass” that looks unremarkable but is distinct enough that your father unconsciously classifies it as in use rather than dirty. Even a tiny cue—a rubber band around the base, a different tint, or a spot you always leave it—can train both of you into a new equilibrium without adding that extra step you’re trying to avoid.

You’re already doing what engineers call reducing transition penalties: keeping the routine intact while quietly adjusting one coefficient in the system.