Not busy at all. Unless it rains than it will be busy. Rain always bring people in the museums.
I went on a Thursday during the day recently, and there was still a good crowd. Did not have to wait to buy admission. Get to any live demos 15 minutes early if you want good seats.
Weekdays is usually less crowd.. but even on weekends, though there is some crowd, we never had to wait for long for any exhibits.. except for parking which can get full during first half of the day on weekends..
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