Exodus 16:14

When the dew was gone up To wit, into the air; or was vanished, there lay a small round thing According to Numbers 11:9, there was a dew which fell before the manna; for it is said, when the dew fell in the night, the manna fell upon it. But it appears here, that there was also a dew upon it, which went up when the sun rose. So that the manna lay as it were enclosed. This might be designed to keep it pure and clean.