... like I'm 5 years old
Ice floats because solid water is lighter, for its size, than liquid water. That may sound odd, because most substances become denser when they freeze. Melted metal, wax, and many other liquids usually shrink into tighter solids as they cool. Water is unusual.
A water molecule is made of one oxygen atom and two hydrogen atoms. These molecules are attracted to one another. When water is liquid, the molecules are constantly moving, sliding, bumping, and rearranging. They can pack fairly close together.
When water freezes, its molecules slow down and settle into a more orderly pattern. Because of the way water molecules attract each other, they form an open crystal structure with extra space in it. That makes ice take up more room than the same amount of liquid water. So ice has lower density.
Density is the key idea. If something is less dense than the liquid around it, it floats. If it is more dense, it sinks. Ice is less dense than liquid water, so a cube of ice rises to the surface and stays there.
This is also why a bottle of water can burst in the freezer. The water expands as it becomes ice, pushing outward against the container.
Think of liquid water like people standing close together in a crowded elevator, while ice is like those same people holding hands in a wide circle. The same “people” are there, but the circle takes up more space, so it is less tightly packed.
... like I'm in College
To understand why ice floats, we need to look at density and molecular structure together. Density means mass divided by volume: how much matter is packed into a given amount of space. Ice and liquid water are made of the same molecules, but those molecules arrange themselves differently.
Water molecules are polar. The oxygen end has a slight negative charge, while the hydrogen ends have slight positive charges. Because opposite charges attract, nearby water molecules form hydrogen bonds. These bonds are not as strong as the covalent bonds inside each molecule, but they strongly influence how water behaves.
In liquid water, hydrogen bonds are constantly forming, breaking, and reforming. The molecules move around enough to fit into many temporary arrangements, some of them quite compact. As water cools, the molecules lose kinetic energy. Near the freezing point, they begin to settle into a more stable pattern.
In ice, each water molecule tends to form hydrogen bonds with neighboring molecules in a tetrahedral arrangement. This produces a hexagonal crystal lattice, the ordinary form of ice found under everyday conditions. That lattice is relatively open: it contains more empty space than liquid water does.
As a result, ice has a lower density than liquid water. At normal atmospheric pressure, liquid water reaches its greatest density at about 4°C. Below that, it begins to expand as it approaches freezing. When it becomes ice, the expansion is significant enough that the solid floats.
This property has enormous consequences. Lakes freeze from the top down, not the bottom up. A floating ice layer insulates the liquid water beneath it, helping aquatic life survive winter. If ice sank, many bodies of water in cold regions would freeze much more completely.
Imagine building water out of Lego bricks. Each water molecule is a small piece with one oxygen side and two hydrogen sides. The shape matters. It is not a straight piece; it has a bent shape, and its ends are attracted to matching ends on other pieces.
When the water is liquid, the Lego pieces are loose in a box. You shake the box gently. The pieces tumble, slide, and crowd into gaps. They do not lock into one permanent design. For a moment, a few pieces connect; then they pull apart and connect somewhere else. Because they are moving and rearranging, many of them can fit into a fairly compact space.
Now imagine the temperature dropping. The shaking slows. The pieces no longer tumble as freely. Instead, they begin snapping together according to the rules of their shape and attractions. But the structure they prefer is not a tight brick wall. It is more like an open framework, with repeating spaces inside it.
That open framework is ice. The same number of Lego pieces now takes up more room than before. Nothing has been added, but the structure has expanded. Since density means “how much stuff fits in how much space,” the expanded Lego framework is less dense than the loose, crowded pieces in the box.
Put that Lego ice structure into liquid water, and it floats because it is the lighter structure for its size. The liquid water beneath it is more tightly packed, so it can support the ice.
This also explains why icebergs show only part of themselves above the sea. Most of the Lego framework is below the surface, displacing enough water to balance its weight, while a smaller part remains visible above.
... like I'm an expert
The buoyancy of ice is a macroscopic consequence of water’s hydrogen-bond network and the density anomaly that arises from it. Under ordinary terrestrial conditions, the relevant solid phase is hexagonal ice, ice Ih. Its structure is governed by approximately tetrahedral coordination: each H₂O molecule participates in a hydrogen-bonding network with four near neighbors, two as donor and two as acceptor, consistent with the Bernal-Fowler ice rules in an idealized description.
This arrangement produces an open lattice. The O—O separations and tetrahedral geometry prevent the molecules from occupying space as efficiently as they can in the dynamically disordered liquid. Liquid water retains substantial local tetrahedral order, but its hydrogen bonds are transient, distorted, and interconverting. This allows partial collapse of the open network, especially above the freezing point, increasing the number density relative to ice.
At one atmosphere, ordinary ice has a density of about 0.917 g/cm³ near 0°C, while liquid water near the same temperature is close to 0.9998 g/cm³. Water’s maximum density occurs at roughly 3.98°C, reflecting a balance between ordinary thermal contraction on cooling and the increasing influence of open, hydrogen-bonded local structures. Below that temperature, expansion dominates.
Floating follows directly from Archimedes’ principle: ice displaces a volume of water whose weight equals the weight of the ice. Since the density of ice is lower, only part of the ice’s volume must be submerged to displace an equal mass of liquid water. This is why an ice cube, glacier fragment, or sea-ice floe protrudes above the surface.
The behavior is not universal across all phases of ice. At high pressures, other ice polymorphs exist, and some are denser than liquid water. The familiar floating of ice is specifically a property of ordinary ice Ih under common surface pressures.