Connect the cylinder and the piston to create a piston mechanism.
Edit the sketch by dragging the piston so that
it fits into the cylinder, click
, and then click in the
graphics area to clear the selection.

In the graphics area, zoom in to the top of the piston:
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To select a sketch entity,
you must clear Zoom to Area and
some of the other tools on the
View toolbar. To clear a tool, press Esc
or Enter, or select
another tool in its place.