How do I crash Linux?

As root, you can do whatever you want, including an accelarated system crash. Try (reconsider if you really want to crash): 
 
cp /dev/zero /dev/mem

As root, you can even erase all the files on your system with a similarly innocuously looking one-liner (don't do it): 

rm -fr /

This is not to say that Linux is an easy crash, but that the system administrator ("root") has the complete power over the system. You can make MS Windows unusable by trashing some files from C:\WINDOWS OR C:\WINDOWS\SYSTEM directory. An important distinctions that makes Linux resiliant is that the user and administrative accounts are separated.  Regular users can touch only the files they own, and similarly the programs users run can only junk user-owned areas, no matter how buggy or malicious a program might be. The separation of "administrative" and "user" accounts adds to system complexity, yet it also makes the Linux system truly multi-user. This is unlike the old MS Windows approach. With the latest version of MS Windows, Microsoft moves more towards the UNIX approach. An old saying comes to mind: "those who do not know UNIX are destined to re-invent it". 

Conclusion: do not use the root account for routine work. Add a regular-user account as your first administrative task and use this account for your experimenting with Linux. Here is how to do it (as root): 

adduser johnbrown
 
passwd johnbrown
 
[enter a good long password with a number in the middle]
 
[reenter the same password without a mistake]
 
exit
 
[login as johnbrown]

The root account is for administration and setup only.  As root, I would not do things/run programs that I do not exactly understand what they do. At least not on a computer that I plan to use for real work. Really. 

My learning path was as follows: 
 
1. Install Linux.
2. Play around, experiment (root and not root, do cool things, setup stuff. I want to be able to predict the behaviour of my system--only then do I know that I understand it).
3. Re-install.
4. Unless I don't have any more time (I never have enough), goto 2. 

Once properly installed on good hardware, command-line Linux is legendarly stable--it can run for months or even years.  As a newbie, you can almost bet that a funny system behaviour results from either your actions as root, or from flakey hardware.