Elasticity is fundamental to cloud computing. An elastic platform can quickly allocate resources to match the demand of each workload as it arrives, rather than pre-provisioning resources to meet performance objectives. However, even serverless platforms — which boot sandboxes in 10s to 100s of milliseconds — are not sufficiently elastic to avoid pre-provisioning expensive resources. Today's FaaS platforms provision many extra, idle sandboxes in memory to reduce the occurrence of slow, cold starts. Initializing securely isolated sandboxes with a POSIX-like computing environment that today's cloud users expect is slow as it requires booting a guest OS and configuring networking.