* Density is key: Whether something sinks or floats depends on its density. Density is mass per unit volume (how much stuff is packed into a given space).

* Water's density: Water has a density of about 1 gram per cubic centimeter (g/cm³). This means that anything with a density less than 1 g/cm³ will float on water.

* Oil's density: Oil is generally less dense than water, which is why it floats. This means that anything that floats on water will also float on oil.

So, there is no solid that can be both lighter than water (float) and heavier than oil (sink) because if it floats on water, it will also float on oil.