running out of gas

Occassionally Ava will get so suddenly sleepy that she will lie down wherever she is and suck her thumb. She did it recently at Bass Pro Shop during an evening errand and most recently at the Des Moines Science Center. She was having fun climbing up on an activity gym. I was watching her run through a tunnel and I saw her run by one porthole but not the next. I peered in the second porthole and could only see one foot tapping up and down. I climbed up into the tunnel and saw this:As I was taking the photo some other kids came up to play and were clearly unfazed by the time-for-nap casualty.