The earth is always earth. The earth will let anyone sit on it, and the earth never gives way. It never lets you go -- you never drop off this earth and go flying off through outer space. Likewise, sky is always sky; heaven is always heaven above you. Whether it is snowing or raining or the sun is shining, whether it is daytime or nighttime, the sky is always there. In that sense, we know that heaven and earth are always trustworthy.