For the first installment of "Ask a Scientist," I will diving into the world of pudding.  But first...

Pudding takes the shape of it's container so it must be a liquid, right?

I went back and read my original "Ask a Scientist" post and I wanted to clear something up first.  I never intended for the post to sound arrogant or snobby.  My only intention was to try to find some new interesting subjects to look into and write an informative post on them.  I am not saying that I know all the answers or that I am a genius.  I just enjoy researching all kinds of science and I also love to share fun science (such as quantum locking)!  So without further delay, let's get back to pudding.  Which is an awesome topic to discuss!

So is pudding a solid or liquid?  Short answer, it is a liquid.  Long answer, it is hydrocolloidial gel.  I'll explain...

Milk is another example of a colloid, because the solid fat molecules are dispersed in the water of milk.

Short answer; if we look at pudding like we were in grade school again and try and determine it's state of matter we would classify it as a liquid.  It has no definitive shape (it takes the shape of the container it is in) but it will not diffuse out of an open container.  So by all accounts it is a liquid.

Long answer; when making pudding there are essentially two phases that make up the pudding.  The first is the powder mix (or raw ingredients if you happen to be a grandma or Paula Dean) that consists of a starch, sugar, flavorings, colorings, and probably some preservatives.  This comes in the form a of solid powder.  Then we have the liquid phase, usually water or milk (which is essentially water for our purposes here).  Now when these two phases are combined and are not completely miscible they form a colloid.  Miscibility is the ability for two things to coexist as one medium (like when you dissolve sugar in warm tea to make sweet tea).  Colloids are when one phase is dispersed within another phase.

Pudding is the same state of matter as hair gel! A liquid dispersed within a solid.

Here, the main two ingredients that determine the overall state of matter for pudding is starch and water.  Because starches have variable molecular weights and consist of chains of sugars, they tend to form amorphous solids, solids without a definite crystal structure and have a little liquid like properties.  Once the water is dispersed within the amorphous solid of the starch, this increases the liquid like properties overall.  This type of colliod is called a hydrocolloid because it specifically uses water.  Lastly, this is considered a gel because the water is dispersed within the solid.  So thus the hydrocolloidial gel!  Which is actually more like a solid.

So to answer the question is pudding a solid or liquid, it is both!  I hope that helps and you can impress your friends at your Christmas parties!  Thanks melpels for the question!

PS - Tapioca pudding is the easiest example of this hydrocolloidial gel that is both a solid and liquid, because there are literal chunks in that pudding, ugh.