Styrofoam floats in water because it is less dense than water. Density is a measure of how much mass is packed into a given volume. When an object is placed in a fluid like water, it experiences an upward buoyant force that opposes the force of gravity.
The buoyant force acting on an object is equal to the weight of the fluid displaced by the object. If the weight of the fluid displaced is greater than the weight of the object itself, the object will float. If the weight of the object is greater than the weight of the fluid displaced, the object will sink.
Styrofoam is a type of plastic foam that has a low density because it is filled with air pockets. Air is much less dense than water, so when a piece of styrofoam is placed in water, the weight of the water it displaces is greater than its own weight. As a result, the buoyant force is greater than the force of gravity pulling it downward, causing the styrofoam to float.
On the other hand, objects that are denser than water, such as most metals or solid plastics, will sink because their weight is greater than the weight of the water they displace. The buoyant force in this case is not enough to counteract the force of gravity, causing the object to sink.
In summary, whether an object floats or sinks in water depends on its density relative to the density of the fluid (in this case, water) it is placed in. Objects with lower density than the fluid will float, while those with higher density will sink.