Quantum entanglement is often called spooky/surprising/unintuitive, but they key question is to understand why.
To understand that, you have to understand why it is fundamentally impossible for the entangled particle pair be in a predefined state according to experiments done e.g. where one is deterministically yes and the other deterministically down.
In other words, why local hidden-variable theory is not valid.
Basically a precise statement of "quantum entanglement is spooky".
Some of the most remarkable ones seem to be:
- Alain Aspect 1982
- Hensen et al., Giustina et al., Shalm et al. (2015): "loophole-free" Bell tests