alert

"a brisk walk in the park"; "a lively gait"; "a merry chase"; "traveling at a rattling rate"; "a snappy pace"; "a spanking breeze";

"an alert mind"; "alert to the problems"; "alive to what is going on"; "awake to the dangers of her situation"; "was now awake to the reality of his predicament";

"caught by a couple of alert cops"; "alert enough to spot the opportunity when it came"; "constantly alert and vigilant, like a sentinel on duty";

"bombers were put on alert during the crisis";

"The empty house alarmed him"; "We alerted the new neighbors to the high rate of burglaries";