Only Pushing forces exist right? I am a third-year high schooler and I remember Michio Kaku once said (in some documentary which I can't sadly find now) That only pushing forces exist. Pulling a door is actually your fingers pushing it from the other side. Gravity is the curvature of space-time so it is pushing it towards earth. The charges probably push by their electric fields. My question is what is the pushing force in magnets,
Thanks.
[Sorry for bad english]
 A: Not all forces push, some can pull, like gravity or the electrostatic force between oppositely charged particles.  What Michio Kaku probably meant is that only contact forces exist: in other words, objects can only affect each other if the are exactly at the same point - they can't exert a force on something far away.  Gravity and electromagnetism might seem to be counterexamples, but you can think of objects interacting via gravity or E&M as exchanging particles called gravitons or photons, respectively, that can't travel faster than the speed of light and "transmit" the force between the two objects.  So when magnets attract, it's because of the exchange of photons traveling between them, and the one magnet is not directly influenced by the other magnet, just by the photons "coming out" of it.
A: In electromagnetism, the force between two electric charges $q_1, q_2$ is
$$
 \vec F = \frac1{4\pi\epsilon_0} \frac{q_1 q_2}{r^2} \hat r.
$$
There are two types of charge which add, in equal quantities, to zero, so we call them "positive" and "negative" charges.  And what you can see is that same-sign charges feel a repulsive force, along the separation vector $\vec r$, but for opposite-charge particles the direction of the force changes sign and becomes attractive.  I can't think of a way to describe the one    as a "pushing" force but the other not as a "pulling" force.
Magnetism is a side effect of electricity; you can actually find all the rules of magnetism by thinking about electricity and special relativity at the same time.  Magnetism gives you a set of rules about electric currents:


*

*Parallel currents ($\uparrow\uparrow$) attract each other.

*Antiparallel currents ($\uparrow\downarrow$) repel each other.

*Skewed currents ($\uparrow\rightarrow$) feel a torque that makes them want to align.


For permanent magnets the "current" is actually a single unpaired electron orbiting each atom, or possibly the "spin" of that electron, but that gets a little ways into sneaky territory.
If you want to think in a field picture, you have to consider the fields produced by both source particles. In that case, what you find is the sources of the fields tending to adjust their locations so that the total energy stored in the field (proportional to field strength squared in each little volume unit) is minimized. I have a hard time calling that pulling or pushing, and it's closest to the modern view.
A: Not all forces are pushing forces.  Some are attractive (pulling), such as gravity.
