I really enjoy the little videos that play when you hover your cursor over the tool icons in the toolbar (otherwise known as "rich tooltips"), but I feel very sorry for the Sponge tool as it doesn't appear to have one!
I realize that the desktop version of Photoshop doesn't have a rich tooltip for quite a few tools, but in the case of Photoshop on the web, this is the only tool not to have a rich tooltip, so it looks kind of clunky not to have one.
Many of the rich tooltips for Photoshop for the web come straight from the desktop version, but I've noticed that a few new ones have been specially created such as those for the Place tool and the Transform tool, so maybe the same team behind those could rustle up a new rich tooltip for the Sponge tool too.
Many thanks!
Phil