...and it's sometimes called the G clef
...because it shows where G is, on one line up
...it's just a decorated version, like in monastery manuscripts!
Similarly the bass clef (F clef) as a capital F, for one line down!
Try drawing the capitals and turning them into the clef (or the other way round is best for the bass clef)...
...everyone loves seeing this done, it's like a revelation!
Here's how...
For the treble clef, draw a capital G with the bottom right line part of the G extending down a bit. Then start drawing the swirl bit of the treble clef over the top of that line bit, follow round the shape of the curve to get to the top, do the circle above the top, and cut down through the line bit and at the bottom of it do the low swirl!
For the bass clef, start by drawing the bass clef itself (the usual way round as if it was on the right hand side of a circle, and don't forget the dots on the right - but don't put the dots too near the clef for this example). Then draw a capital F by joining the dots to the clef using horizontal lines, and the vertical line of the F follows the curved line of the clef down. The swirl to the left on the clef is just decoration which better shows where the F note is, on one line down, just like the starting swirl of the treble clef!
Happy demonstrating!
P.S. In Hal Leonard's Student Piano Library, Piano Lessons Book 1, it says that the signs come from the "old-fashioned" G and F (and shows pictures on pages 38 and 40) but it doesn't say whether they were capitals or not. Whatever, the modern capital seems to be a descendant of these signs, and shows the principle well when a treble or bass clef is drawn over them.