:Undulate and fartlek (yuk!) are characterized by a cycle, a variation, an alternation,
an oscillation. Incidentally, my dictionary gives "swing" as root menaing of "oscillation".
I wonder if the ancients built swings for children's recreation?

In reply to:

---

I wonder if the ancients built swings for children's recreation?

---

that's a interesting thought, Bill. "toys of ancient times"...

How do you like to go up in a swing,
Up in the air so blue?
Oh, I do think it the pleasantest thing
Ever a child can do!

Up in the air and over the wall,
Till I can see so wide,
River and trees and cattle and all
Over the countryside--

Till I look down on the garden green,
Down on the roof so brown--
Up in the air I go flying again,
Up in the air and down!

I wonder when the see-saw was invented.

Beware of the man with a series of two cases.
"lek" is interesting in its own right:
n.
an area where certain male animals, as the male of the prairie chicken, gather to perform their courtship displays