Matthew Poole and continuators on Exodus 16:21

21 And they gathered it every morning, every man according to his eating: and when the sun waxed hot, it melted .

To wit, as much of it as was left upon the ground. This was not from its own nature, which was so solid that it could indure the fire and was bruised by a pestel, but from Gods wise providence, partly that it might not be corrupted or trodden under foot or otherwise abused and so despised, partly that it might not remain there to tempt any of them, to gather more of it than they should; and partly that all their stock of provision being wasted, they might be obliged to the more entire dependence upon God. And this is here mentioned as a reason why they gathered it in the morning.