Because of -- because of the mask? >> that's correct. that's the primary intervention. these schools didn't do widespread-screening testing. they didn't do widespread ventilation changes. they simply had universal masking. they had very high compliance with that. we had substudies in several districts, which showed a compliance rate greater than 90% in the mainstream curriculum and we're getting ready to submit that for publication here, later-this month. but, yeah, the primary intervention, masking. >> do you know what it would be without a mask? i mean, what -- what -- if a child with covid ...