Stripe accounts have both a live and a test mode. The two modes exist in parallel; you can always try something out in test mode, even after you’ve gone live with your site. Test mode differs from live mode in that you can’t use real credit cards, only test card numbers. Every Stripe interaction can be in either live or test mode, simply by using live or test API keys. Emma follows the standard development route by starting in test mode and, when she is satisfied with the work, swapping her test keys for her live keys.