40th Birthday Gifts For All STEM Math Equation T-Shirt The best 40th birthday gift is a funny 40th birthday gift. Celebrate with a milestone birthday gift. 100% cotton (preshrunk), Sport Grey is 90%/10% cotton/poly. This shirt has seamless double needle collar with double-needle sleeves and hem with a taped neck and shoulders. The design is printed with new age printing technology, direct-to-garment. It is printed with a water-soluble and eco-friendly ink. It is cured with a heat treatment process to ensure the color-fastness and lasting durability of the design. Authentic Birthday Gifts For All  design printed with pride in the United States. This is not an unauthorized replica/counterfeit item. This is an original inspired design and does not infringe on any rights holders rights. Words used in the title/search terms are not intended to imply they are licensed by any rights holders.