The primary mechanism involved with microwave cooking is the interaction between the alternating electric field component of the 2400 Mhz microwave electromagnetic field and the polar molecules of the water content of food. This gives the water molecules rotational kinetic energy. The rotational kinetic energy is then randomized by means of collisions between water molecules, increasing the translational kinetic energy of the molecules. That, in turn, results in an increase in the temperature of the food thus cooking the food.
Its a common misconception that microwave ovens "heat" food. Heat is energy transfer solely due to temperature difference. Microwave ovens don't cook like a conventional oven which uses heat. The energy transfer to the food involved with microwave ovens is work. Namely, the work that the alternating electric field performs on the water molecule dipole.
Are you saying that the microwaves increase the rotational energy of a
molecule the wave enters? Why does the microwave increase the
rotational kinetic energy? I am very curious how microwaves that enter
a molecule increase that molecule’s rotational energy.
There are two ways of looking at it, one using the wave (classical) model and one from the particle (quantum) model.
The diagram below is perhaps a crude way of showing how the electric field component of the electromagnetic wave interacts with a water molecule inducing rotation. The water molecules is a an electric dipole owing to the spacial arrangement of the two hydrogen at one end and one oxygen atom at the other. It is overall electrically neutral, but is positive at one end and negative at the other forming a dipole, as shown.
At the electromagnetic wave propagates past the molecule, the electric field component alternates in polarity. When it does the molecule aligns itself (rotates) with the field due to the attraction and repulsion forces. That gives the molecules rotational kinetic energy.
Particle (Photon) Model:
In the photon (particle) model, the quantum energy of microwave photons is in the range 0.00001 to 0.001 eV which is in the range of energies separating the quantum states of molecular rotation and torsion. So in this model you can view the transfer of microwave electromagnetic energy in the form of photons absorbed by the molecule dipoles causing them to undergo rotation and torsion.
For a more detailed description of the interaction of electromagnetic radiation with matter, check out the following site: http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html
Hope this help.