Explain · Like I'm five version
The periodic table in terms of a school seating plan
The periodic table is just a big school seating plan for all the tiny bits that make up everything in the world.
Each box on the table is like a desk where one type of element sits. The teacher puts the rowdiest elements, like the ones that pop and fizz in water, all in one row together. Elements that are quiet and do not like to talk to anyone else get their own special row at the very end. The teacher knows exactly where to put them based on how many tiny invisible marbles they have inside.
When elements sit near each other, they sometimes like to swap things or hold hands to make new stuff. A shy little gas might sit next to a grumpy metal and suddenly they are stuck together as a piece of salt for your chips. Some elements are very popular and have many friends, while others prefer to sit alone and stare at the wall. It is exactly like how your teacher tries to stop you from sitting next to your best friend so you actually finish your work.
If an element has too many marbles, it might get sent to a different table to keep the classroom balanced. This keeps the whole room from exploding during science time. It is a very tidy way to make sure that everything stays exactly where it belongs, from the water in your cup to the shiny frame on your bicycle.
Where the analogy breaks downElements cannot move seats to be closer to their friends or leave the room for lunch.