Dark... it was intended as a highly simplified scenario. Since you have knowledge in this area why don't you tackle this one.
Ok, put simply;
Firstly we need to establish that plates move over the soft mantle causing convection, and rediculous amounts of pressure.
Depending on the type of plate and its interaction, mountains can and will form . The Himilayas are a result of two continental plates smashing into eachother. To refer to the bus example, its like the buses hitting, and then one buses front wheel ending up on the roof of the other bus. In regards to continental plates, its often hard to explain why one plate was pushed above the other because they're they same density and unlike the oceanic plates, we cannot conclude that bouyancy was the reason for uplift.
The three major tectonic actions are:
Transform actions: Plates grind together and no plate is pushed up or down. Since there is a lot of stress and convection, potential energy which is built up as a result, is released as strain (elongation or the stretching of a solid). As we know, rocks aren't too good at stretching in a lot of cases, so this can cause problems depending on where it happens. The San Andraes fault line experiences a lot of transformation along its length.
Rifting actions: As i said before, see floor spreading is an example of rifting. Plates are pulled apart causing a void usually filled with molten magma from the mantle (as you would expect). Hotspots (usually known as volcanos) are created here using kietic energy caused by convective currents beneath. Hotspots only occur when the pressure is great enough to break through the lithosphere. Iceland has a huge divergance in the
Alfagja Rift Valley. Its pretty cool looking File:Bridge across continents iceland.jpg - Wikipedia, the free encyclopedia
Convergent Actions: This is where the continental and oceanic plates crash into eachother. Because the oceanic plates are more dense than continental plates, the oceanic plates sink under causing mountains due to their lack of bouyancy.
If two continental plates crash together, mountains are formed (like the Himilayas) because one plate doesn't sink below the other (to put it simply). Massve uplift due to convective kinetic energy and pressure is what forms mountians as low density material is pushed between the plates from the mantle.
However, in practice it doesnt quite work like this. In an ideal world thats how plates interact.
So in conclusion. Without adding complicated factors that you would need a PHd in geology to understand, colliding continental plates are just one way in which mountains form, but are by no means the only. Maountains also do not hold the plates together, but in most cases, such as the Himilayas, collisions between the plates keep the mountains growing rather than breaking them apart. Thats why the himilayas are growing.