A crown is a bonus that you can add on to the boxes you have. The crown act as an extra star for a random part that you get from the box. Each box can only have a maximum of 3 crowns, and each part can only have 1 crown. Crowns are obtained by getting a winning streak. Each crown can only be obtained after a winning streak of 5 is achieved. In some special cases where if your stage allows you to get 5 stars on a part, a crown can boost that 5 star part into the next tier. For example, if you are at stage 4, you can get 5 star wooden parts. If you opened a box that has a crown, and also get a 5 star wooden part that gets the extra star from the crown, the part will be upgraded into a 1 star metal part.