Walk me through how you would design a scalable web application on AWS.

I would design a 3-tier architecture: 1) Web tier: CloudFront CDN + ALB + EC2 Auto Scaling Group in public subnets, 2) Application tier: EC2 instances in private subnets with Auto Scaling, 3) Data tier: RDS Multi-AZ with read replicas. Include ElastiCache for caching, S3 for static assets, and CloudWatch for monitoring.