How should we classify tooth paste and shampoo when teaching 2nd graders? I mean they are only taught about solids, liquids and gases so which category do these fall in? I know they are colloids in the real sense, with a solid/liquid dispersed phase and a liquid dispersion medium but how should we e

Liquids are things that flow.  Even if they flow very slowly, they are still liquids.  I used to work for a company that made high-viscosity silicone fluids.  You could actually pick them out of the jar and squish them in your hands!  But when you put them back in the jar they would--slowly!--flow until they filled the bottom of the jar.  I would suggest that you tell your second graders that if it gurgles, sloshes, or oozes, it is a liquid.

I asked this question to my teachers when I was 9 and they gave me the answer that they are liquids, with the reasoning that they flow into the shape of the container they are in, just very slowly. I think as long as it doesn't conflict with what is on the syllabus in a way that might confuse them, then it's fine to explain it any way you like.

"Non-newtonian solid/liquid"

They should all be familiar with foods like mashed potato, ice cream, etc. where you mix liquids and solids to form pastes.

Lots of good answers above -- but we're talking 2nd graders. And I have one, and I know his attention span, and how much of an answer he wants. And I've done a lot of guest science presenter demos, including gas/liquids/solids!, for both my kids' classes'. So the first answer (Matt Harbowy) is beautifully accurate -- but I'd give it to someone in at least 5th grade, and then only if they were really into the topic. 2nd grade? Here's the answer: "they're mixtures of stuff! There's powdered stuff (that's solids!) in some liquid stuff -- that makes kind of a "paste". More powder (solids!) - it's thicker! More liquid, it's thinner! Think of mud: it's dirt plus water, right? When it dries out, it's just dirt! If you add a bunch of water, it's just dirty water!" "Toothpaste and shampoo are just stuff-in-liquid, too. Toothpaste has some gritty stuff to polish your teeth, like sandpaper only softer; it also has some things that are good for your teeth and make them shine and keep you from getting cavities -- and some things that make it a little thicker. Shampoo is also liquid -- mostly water ('aqua'!) with other stuff to clean your hair, and to make the shampoo smell good and color it and make it thicker." ---------------------- Try to explain colloids, suspensions, gels, etc. to a 2nd-grader, in the same discussion? -- and good luck keeping their attention. The old oobleck/cornstarch/white-glue stuff is fun -- but leave it out of the initial gas/liquids/solids discussion as it'll only go off into the weeds (trust me, I know this from very very direct experience). I've answered these questions before: (a) wait until they ask them!, and (b) give them just enough info. Shampoo is basically a liquid (def'n: fixed volume, variable shape). Toothpaste actually fits that def'n too although you can say "it's kind of an in-between combination" which satisfies most 2nd-graders, especially if you say "if you want to know more you can stay after and I'll explain it to you!". At that age: slow-flowing stuff, colloids, suspensions, gels? -- most 2nd-graders don't really care yet, any more than they want to know that plasma is a 4th state of matter made of superheated ions, or about superfluid CO2 beyond the triple-point. 60-second lesson: "Gas: variable volume and shape; Liquid: fixed volume / variable shape; Solid: fixed size and shape (yeah, powder/sand, now shaddup, smart kid! >;-) Some liquids are 'thicker' than others. Later on, we'll learn that there are combinations of these 3 things ("phases"), like foams, colloids, suspensions, aerosols, emulsions, solutions, sols, gels, mixtures, etc. -- but that's for later!"

I would argue that it'd be difficult to classify those things with most adults.You're actually talking about "https://en.wikipedia.org/wiki/Bingham_plastic" which most people have never heard of.If I had to explain them to second graders, I'd say that "some things, like toothpaste and mashed potatoes, are solids if you leave them alone, but liquids if you push on them.  They're kind of a special case that can act like either a solid or a liquid depending on where they are."As for shampoo, I'm inclined to just call that a liquid.  It'll likely flow well enough if you give it a minute.  (But maybe you use an interesting shampoo!)

The phase in which the particles are dispersed becomes the dominant phase of the substance itself. Or just tell them, these are mixtures of more than one state. The first method will essentially simplify it for them and the second answer will force them to think. Your pick :)

Solids are substances that have a rigid shape. Liquids take the shape of the container they are kept in. This is the general definition of solids and liquids that is taught in primary classes. So, as per this, toothpaste and shampoo have to be put in with liquids.

I would refer to toothpaste and shampoo as 'semi-solids', having properties intermediate between solids and liquids. An alternative term to use is 'gel'.