Asymetrical encryption solves this, here is my attempt to do an ELI5:

Adam want's to send a chat message to Ben, but want to do it securely, so they use a special program on their computers.

When the Adam's program first reaches out to Ben's computer, it asks for an unlocked padlock, this is a padlock that can only be unlocked by Ben's program.

Adam's program takes the padlock and crafts a new special series of padlocks that only Adam's program can unlock, which it put's in a box and locks it with Ben's padlock.

The box is sent to Ben's program, the program unlocks the box and creates it's own special series of padlocks that only Ben's program can unlock, put them in a box and locks it with Adam's padlocks.

The box is then sent to Adam's program, and is unlocked.

This now means that Adam's program can put messages to Ben in a box, lock the box with one of Ben's special padlocks and send it on it's way knowing that only Ben can unlock the box and read the message.

Likewise, Ben can also send messages in boxes locked with Adam's padlocks and know that only Adam can unlock them and read the message.

Added to this is the fact that messages from Ben can be verified as having used the special padlocks Adam sent to Ben, as else Adam's special key wouldn't fit the padlocks given to Ben.

In reality the padlocks are keys to lock a message, and the above text describe a secure key exchange.