<i>&quot;Illness is often caused by tiny lifeforms, invisible to the naked eye&quot;</i><p>I think this is the most important thing to learn, because it would spare the future from a lot of suffering and give future people leisure time to think about the world and discover things for themselves. However, I don&#x27;t like that this is a bit of knowledge that doesn&#x27;t easily allow humans to verify it.<p>Instead, I would tell them that one can see many tiny things some of which make us ill, but not the tiniest of them, by staring through a tiny water droplet.<p>Item #2 would, I think, be a statement on the periodic table that can easily be verified using minimal technology, again with a hint that there are even smaller things.