次序

次 = 冫 + 欠 : Mnemonic symbol: the cube, as in words involving 三次 relate to "cubic equations" or "cube (third power)". The King of Chu (c) pulls a popsicle (冫) out of the cube (次) in the space station's bathroom (Ø4) and leaves an IOU note (欠) on the cube.

序 = 予 + 广 : Due to an emergency Xu Xian (xu) and Zai Yu (予) both have to leave the space station from the bathroom (Ø4) and parachute (广) back to earth. They just cannot decide on the sequence (序) of who's going first.