Don't forget to specify with soap and to do it after bathroom visits and before meals, otherwise they'll just wash their hands once per month and use dirt to wash with.
Telling them how a steam engine works.
That would start the industrial revolution earlier and it would end up speeding us up to a more advanced and better future... or to an early extinction by global warming... hmmmm
Maybe explaining an electricity generator would be a better gamble, but it may be very hard to make one back then...
The Romans had steam engines. But they couldn't be used for anything but opening temple doors to impress people because they didn't have the manufacturing tolerances to seal the steam chamber properly, nor the metallurgy needed to pressurize it without bursting.
That tech only became available much later during the industrial revolution.
AFAIK even that wouldn't have been unsolvable problems for Greeks and Romans.
However why put all the effort into this machinery when you can simply put more slaves to work? One driving factor for the Industrial Revolution was the issue of having to pay people actual wages instead of being able to force them to work. This added incentives to reduce manual labor and replace them with something owners can force to work without paying it: Machines.
The answer would probably be something sensible, within our ability and something we would be capable of.
We already know how to do this .... we as a collective global civilization just actively refuse to do any of it unless we were forced to, either by war or by the realization that it will mean our extinction.
It's not that we are a thousand years dumber than our descendants .... we are just as intelligent and insightful as our ancestors a thousand years ago ... it's just that our short sightedness and greed gets the best of us.
In many ways we are still a lot like our prehistoric ancestors from 100,000 years ago who believed that our family should control all the unlimited amount of bananas in the jungle.