Heterogeneous Mixture
A heterogeneous mixture has parts that are not evenly distributed, so different samples have different compositions and you can often see the pieces.
Definition
A heterogeneous mixture has parts you can see or that are not spread evenly, so one scoop might have more of one thing than another scoop. Its composition is non-uniform because the components remain in distinct regions or phases; suspensions such as sand in water settle over time, and many heterogeneous mixtures can be separated by simple methods such as filtering, sieving, or decanting. Formally, it contains two or more phases with boundaries between them, so intensive properties differ from region to region. Suspensions have particles large enough to settle under gravity, while colloids (emulsions, aerosols, gels) have dispersed particles from about $1$ to $1000\text{ nm}$ that do not settle but do scatter light, and emulsifiers stabilize otherwise unstable mixtures by lowering interfacial tension.
Example
A bowl of cereal with milk is heterogeneous, and so is a salad or a handful of sand and pebbles. Muddy water is a suspension: shake it and the soil spreads out, let it sit and the soil settles on the bottom, and pouring it through filter paper separates the two. Mayonnaise is an emulsion of oil droplets in water stabilized by lecithin from egg yolk, whose molecules have a polar head that faces water and a nonpolar tail that faces oil; without the emulsifier the oil and water would separate within minutes.
Key Insight
"Hetero" means different, so if different spoonfuls look different, the mixture is heterogeneous. Oil and vinegar salad dressing stays heterogeneous no matter how hard you shake it, because oil does not dissolve in water; it looks blended for a moment and then separates into layers again. Granite is a heterogeneous mixture of quartz, feldspar, and mica, each with a different melting point and density, and geologists read the crystal sizes to infer how fast the rock cooled, so heterogeneity is data, not just a classification.