[sliverstorm]

The idea isn't "how you cross a stream: swim across the stream". Read all the instructions, and you see at the end:

... those unable to swim are safely passed over.

Knowing how to swim was not a universal skill, much like knowing how to read or write.

[pmjordan]

You may be surprised to hear that knowing how to swim is still far from a universal skill.

[sliverstorm]

I know, but I think it is more universal than it used to be.