I recently came across this piece:
What piece is it, and how can it be used?
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The part is Window Round 3 x 3 x 2 (73878).
It was a new mold for 2021, and one of its first uses was as windows on Andrea's House (41449).
This piece can be used on buildings anywhere you'd like a curved window. It is easy to imagine how it could have been used on earlier sets, such as the "curved" windows on the Pet Shop:
This part introduces opportunities for curved windows, but it continues to use existing window glass. It uses the 1x2x2 glass (60601):
This means that the part is also compatible with other window panes, such as the 1x2x2 bars:
These parts are paired together to create the curved window seen in Chamber of Secrets (76389):
Like most parts, there's plenty of ways to use it. It shows up as fences in Heartlake City Park (41447):
It even gets used to create a water wheel in Medieval Castle (31120):
I played around a little with this part at microscale and put together a Mars research station based around the idea of round pods: