How does a blockchain stay secure if one node misbehaves?

Any disruption or misbehavior by a single node can be isolated by the rest of the network, leaving the system intact. Because every node independently maintains a copy of the shared data and validates interactions using cryptographic techniques, a single compromised or offline device cannot corrupt the whole network. This is what "trustless" means in practice: you don't need to trust any one participant, because the network's structure catches and isolates bad actors automatically. For a beginner, the takeaway is that a healthy blockchain becomes more resilient the more independent nodes participate in it.