Here's why:

* Solubility: Solubility is the ability of a substance (solute) to dissolve in another substance (solvent). Water is a good solvent for many substances, but not all.

* Polarity: Water is a polar molecule, meaning it has a slightly positive end and a slightly negative end. This makes it good at dissolving other polar molecules and ionic compounds (like salts).

* Non-polar substances: Non-polar molecules, like oils and fats, don't dissolve well in water because they don't have the same charge attractions.

* Other factors: Solubility can also be affected by temperature, pressure, and the presence of other substances.

Examples of solids that do dissolve in water:

* Sugar

* Salt

* Baking soda

Examples of solids that don't dissolve in water:

* Sand

* Oil

* Plastic

It's important to note that some solids may only dissolve a tiny amount in water, while others dissolve completely.