Dihydrogen Monoxide!

Source - Reddit

OMG, that solution is so basic.

No, not basic, basic.

Outdated Periodic Table

 

Source - XKCD

That would be so much easier to memorize.

As you can see if you look at the timeline of the Big Bang, that's about right that half an hour after the big bang, the only elements would have been hydrogen (75% of the universe's mass), helium (25%), lithium (trace amounts), and radioactive beryllium.

Of course, I'm not sure what this periodic table could've been printed on - or by whom, but that's not the point.

The rollover joke - Researchers claim to have synthesized six additional elements in the second row, temporarily named 'pentium' through 'unnilium'. - is also outstanding.

Freezing liquid nitrogen

I've done that.

Quite a few years ago, in class, I had some liquid nitrogen thanks to the parent of one of my students, and I was able - with the help of a vacuum pump - to repeat this demonstration in class.

I'm not sure my students were nearly as amazed as I was, but it's a singular occurrence for me.

Very cool...(pun intended)...

How To Make Drawings Float With A Magic Water Marker

That's pretty cool, man.

It's no Animator v Animation or anything, but it's a neat way to show that polar and nonpolar substances typically don't mix.