Portal/Chamber 10

First room
Chamber 10 is where Portal starts to become increasingly more challenging, as you move from the learning to the testing stages of the game. This chamber is where you get to apply the laws of physics – specifically, momentum – to the game, and use your downward momentum going into a portal to cross horizontal gaps.

"Momentum – a function of mass and velocity – is conserved between portals. In layman's terms: speedy thing goes in, speedy thing comes out."

- GLaDOS

The first room is pretty simple; just create a portal in a wall, then walk into the wall from a little distance away. Your forwards momentum will carry you out of the fixed orange end of the portal on the panel onto the steps, and then you can go into the next room.

Second room
The second room is not converting horizontal momentum to horizontal momentum; it is converting downwards momentum to horizontal momentum. There is a fixed orange portal end on a panel above a drop to a floor below. Place a portal on the floor at the bottom of the drop, then fall into it. Your downwards momentum will carry you far when you exit the orange end of the portal, and you can go into the next room.

Third room
The third room takes it a step further, and has you climb up to a higher level using a two-step process converting downwards to horizontal momentum.

There is a fixed orange end of a portal in the floor at the bottom of a drop, and some panels extending from the wall behind you. Place a portal on the panels (it doesn't really matter where, but as a general rule, the higher the exit portal is, the further you'll go), and fall into the orange portal end. You'll exit on the second level.

The panels will move forwards so that the next jump is possible. Once they've finished moving (portals can't go on moving surfaces), place a portal on the panels again, and then fall from the higher height of the second level back into the orange portal end. You'll land on the third level, and be able to exit to the chamberlock.