These Tamper Evident Envelopes feature flaps with patented tamper indicating panels. Once flaps are raised, the words "OPENED" appear in red and cannot be repaired to hide evidence of tampering. Water-resistant and moisture-resistant envelopes are made of DuPont Tyvek so they won't rip, tear or puncture. Design is much stronger and more secure than traditional paper envelopes. These envelopes are lightweight to save on postage costs and easy to use. Simply peel and seal.