doorway

Butler said he’s surprised by how many aging homeowners don’t think ahead about safety and maintenance, such as having entryways with no steps, curbless showers or wider doorways should they need a wheelchair someday.

Just as the frog must find the right opening, an electron also needs to locate a specific "exit," known as a "doorway state."

She stepped through the doorway and switched on her headlights.

"They would awaken sometimes at night to find her standing silently in the doorway, watching them sleep - 'Oh, you're awake?' - with a knife in her hand," he writes.

The rules read like a brick wall and function like a doorway.