1. Observe the edges: Observe the edges of playing cards carefully. The edge is smooth, and there is no obvious gap and layering. This card has two sides.
2. Check the angle: tilt the playing cards from different angles and observe the edges. Look at both sides. This card has two sides. Seeing three sides is three sides. You can see all around. It is four-sided.
3. Use light: put the playing cards under light, light or sunlight. Observe the reflection of light at the edge. You can see two reflection lines, one on each side. You can see three reflection lines, so it is three sides. You can see four reflection lines, which are four sides.