pitfall
坎 centers on a hidden danger in the ground: a pitfall or hole that traps the unwary.
坎 combines 土 (earth) and 欠, which likely contributes the sound and the idea of a hollow or lacking space, together suggesting a pit in the ground.
The left side is earth 土, and the right side 欠 looks like a person yawning or a gap. Picture a hidden gap in the earth that opens like a yawning mouth: a pitfall.
For カン, imagine a can hidden in a pitfall: can -> カン, and stepping on the can gives away the trap.