Size
I referred to the size of everything to explain how small bacteria and viruses are.

How do bacteria and viruses work
Viruses hijack the machinery of a cell to force it to make endless copies of itself. In the process, the invaded cell can be destroyed. Bacteria make you sick by producing toxins, destroying cells and using up the bodies resources.

How does the body defend itself
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Vaccines
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Disease spread
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difference between viruses and bacteria

Images of various bacteria and viruses

Time-lapse movie of bacteria growing

Where did viruses come from?

Animation of a virus invading a cell and reproducing

Macrophage in action

B and killer T cells
B cells produce antibodies
antibodies attach to antigens
the killer Tcells move in and destroy the antigen
antibodies then remain in circulation
antibodies can also bind to toxins

Scientists have come a long way in estimating the number of cells in the average human body. Most recent estimates put the number of cells at around 30 trillion. Written out, that's 30,000,000,000,000! The same number (give or take) of bacteria live in and on our body. Many bacteria are helpful, like the ones that help you digest food, help you fight disease and provide nutrients to your body. According to the National Institutes of Health (NIH), fewer than one percent of bacteria can actually make you sick.

Vaccination, herd immunity and the protection of communities:

• Interactive herd immunity simulation (square)
• Interactive herd immunity simulation
• Measles graph, before and after vaccines
• Herd immunity animation
• Vaccination rates by state
• Smallpox was eradicated from the world by vaccination
• Polio eradication is next

A looming threat is antibiotic resistance, where fewer and fewer currently available antibiotics are effective. Here is a top 10 list of antibiotic resistance

There is a long list of things that spell the end of the world. There is a class of events with a known date: The time when the Sun will swell up and swallow the Earth, or the time before that when the oceans boil away and the atmosphere is blasted off. Then there is a class of events that may destroy the Earth, but we can't tell when, if ever. These include a hit by a large asteroid or comet, or getting zapped by a nearby gamma-ray burst. Then there are things that will cause a big mess, but will not destroy the entire Earth, such as the next eruption of a supervolcano, or a nuclear war. Then there is the unfolding global warming disaster. It will kill of a large fraction of all species on earth, though not humans and cockroaches, but my country of birth will be gone in a few centuries. Not exactly the end of the world, but certainly the end of the world that I grew up in.
Long after the Earth is gone, the Universe itself will come to an end, though this may take different forms, from a Big Rip to a more quiet heat death.

• List of apocalyptic events
• Timeline of the future
• R.E.M.: It's the end of the world as we know it (1987)

There was a question about caves, so I looked up a few things. Luckily, here in New Mexico, we have several cave systems, and many of the 6th-graders had been to one or more of them.

The most famous one is in Carlsbad, an example of an epigenic cave system. Another famous set of caves are in nearby Bandelier National Monument. These are man-made, dating to the 1100's, made by the ancestral puebloans. We also have lava tubes that you can go into. One even has ice in it year-round.

We talked about stalactites, stalacmites etcetera, and of course snotites or cave slime.

• Los Crystales cave, Naica, Mexico
• animation of limestone cave formation
• Cave types
• Caves in New Mexico
• Lava tubes at El Malpais
• Ice cave at Bandera Volcano
• Cave animals
• Troglobites
• A cave on Mars
• Stinky, hypogenic caves
• Snotites

Lots of history.

• First you need to invent the wheel
• Leonardo Da Vinci's spring-powered cart
• horse-drawn carriages
• Power after musclepower:
  • Steam:
    1769: Cugnot tractor
    ...and many more since
    but how do steam engines work?
  • Electricity
    Electric cars dominated in the late 1800's
    Wood's 1902 Phaeton
  • Fuel: internal combustion, using gunpowder, hydrogen, dust, coal gas, kerosene, gasoline.
    How does an internal combustion engine work
    History of the internal combustion engine
• Car history timeline
• Geschiedenis van de auto

A demo! I made a little contraption to show the difference between internal and external combustion. It's a thing to launch soda bottles with either air pressure (on the right, the bike pump and the red button), demonstrating the external case, or to do internal combustion, do the alcohol rocket trick, using a rubbing alcohol and an Oudin 'hand-held tesla' coil I found long ago to make the spark.
First I used the pump to supply external pressure to launch the bottle, and then I used alcohol and a spark to launch it. One of the kids suggested we do BOTH. So I put some alcohol in, she pumped the bottle up a bit and we hit the button and the spark at the same time. This time, the bottle put a dent into the classroom ceiling. Success!