In short, one word: heat! Also known as thermal energy. You've probably experienced this first hand eating a piece of ice or watching it melt on a hot summer day. Now the question is, why is that so?

The basic idea is that all materials have some sort of interaction or bonding between each of the species (e.g., atoms, molecules). When you heat something up, you give the atoms or molecules more energy to move around. At some point, there is enough thermal energy that the interaction between the atoms or molecules would be disrupted, and your material changes phase (e.g., melts). Let's see this in action...

Ice is the solid phase of H₂O. Although we call various phases of H₂O ice (solid), water (liquid), and vapor (gas), they are all composed of the same atoms, namely two hydrogen atoms and one oxygen atom. Atoms are the building blocks of all matter. An atom itself consists of a nucleus with protons and neutrons surrounded by electrons. Protons are positively charged, neutrons have no charge and electrons are negatively charged. Protons and electrons are electrostatically attracted to each other. If you've ever played with magnets before, you will have an idea what this attractive force is like (although for magnets the force is of a different kind, but the idea is the same).

Here is approximately what a carbon atom is composed of. The nucleus containing the protons and neutrons is much smaller than what is shown and how the electrons fill around the nucleus is quite complicated, so the picture is not physically accurate.

Nevertheless it is useful for explaining certain phenomena.
Hydrogen and oxygen are similar, but have different numbers of protons, neutrons, and electrons. For a neutral atom, the number of electrons and protons are the same. These are captured with the periodic table , which looks like it is arranged in a weird way, but is actually one of the most important tools we use in the research I do!

Here is what a water molecule looks like (i.e., when two hydrogen and one oxygen atom come together to form a molecule).

If you were to look at the periodic table, you would find that hydrogen contains 1 electron, 0 neutrons, and 1 proton while oxygen has 8 of each. Because oxygen has so many protons, its nucleus has a much larger positive charge compared to the nucleus in hydrogen. This means that the oxygen atom draws in the electrons from the two hydrogens a little closer to itself. This makes the oxygen atom slightly negative and the two hydrogen atoms slightly positive. What arises out of this uneven distribution of electrons is something known as hydrogen bonding, in which hydrogens of one water molecule are attracted to the oxygen of other water molecules. You might also notice that the hydrogens are squished together (we call it a bent configuration).

At this point, you might be wondering what this all has to do with ice melting. It turns out it is this hydrogen bonding and bent configuration that gives rise to a lot of interesting properties to water. Ice is a solid, but is less dense than its liquid form. This arises from the bent structure (and the hydrogen bonding to a certain extent), which forces the water molecules to adopt particular orientations (i.e., directions) when frozen. It is also why snowflakes have very particular symmetries (like the ones in this cool photo gallery of snowflakes; pun intended).

When ice is melted, you give the water molecules enough energy to wiggle out of these particular orientations. If you heat the water further, you completely disrupt the hydrogen bonding, and you get water vapor.

It turns you can also change the melting point (and boiling point) of ice, and pretty much any other material. There are many more variables you can change to do this, the most common being pressure. That water boils at 100 C or freezes at 0 C assumes that it doesn't have impurities and we are at atmospheric pressure. But this doesn't necessarily have to be true!

Hope this helps!
Best,

This answer relies on a related question asked a few weeks ago "How does water and air act like a heat reservoir? ".

Heat is what makes any material melt. Like everything on earth, ice is made of atoms. Many many atoms of Hydrogen and Oxygen combine together to form ice. These atoms are stuck together by a force of attraction between the atoms. But, when one supplies energy to ice, the atoms get excited and try to move around a bit. If the energy supplied to ice is large enough, the atoms that are moving can overcome the attractive force between each other, and break free. When atoms break free, ice turns into liquid water. In other words, ice melts.

If the supplied energy is even larger, then atoms completely move away from each other turning liquid water into water vapor.

Ice is made of water molecules which take different shapes depending on the temperature. At high temperatures, the water molecules have a lot more energy and can move freely through the air which is what we call steam. At room temperature the molecules have less energy so they pack tightly together in liquid water. Finally, at low temperatures, the water molecules don’t move as much anymore and form a solid crystal which we call ice. So ice melts when the water molecules get enough energy to move around more which allows them to become liquid water.

Ice and other materials are capable of holding energy in the form of heat. If you put an ice block in a hot environment, it will absorb some of that heat energy. If the ice absorbs enough heat, it reaches a point where it will melt into water. This is its melting point, which is usually given as the melting temperature (32 degrees Fahrenheit). Water is more energetic than ice, meaning it is holding more heat than ice.

To explain WHY this melting occurs, you need to know that materials like ice are made up of many tiny, identical building blocks called molecules. When molecules absorb heat energy, they vibrate. The more they are heated, the bigger the vibrations get until they break apart from each other and form a liquid. This is why a liquid has less structure than a solid, and gets, well...melty!

Ice melts because of either warmth or pressure. Above 0 degrees Celsius (32 degrees Farenheit), ice melts to become water. Enough pressure can also melt ice (this is how ice skates work)

Water can exist as a solid (ice), as a liquid or as a gas, depending on what is the temperature and the pressure.

When we take ice cubes (solid) out of the freezer to the room, we are taking them from a very low temperature to a higher one. Eventually, the temperature of the ice cube has to reach the temperature of the "infinite" temperature "bath", the room. So, the ice absorbs energy, in the form of heat. The water molecules are getting warmer, they have more kinetic energy, they translate and vibrate more and they don't want to stay nicely organized as a solid. So, they disorganize and the ice cube becomes a liquid.

Another way to melt ice is to increase the pressure. Water is quite atypical in this respect. Usually, if we want to melt something you would think we want to decrease the pressure, to allow the molecules to move, and not be tightly packed. But with water it's the opposite. In the ice phase, the molecules are well organized with specific distances between them, prescribed by the hydrogen bonding, and there are specific gaps between them. If we increase the pressure, instead of packing them better, we disorganize their structure and it melts and we achieve this at a fixed temperature. At even higher pressures this thinking breaks down, like for ice found on glaciers.