Spherically symmetric solutions with a flux of electric and magnetic fields in Kaluza-Klein gravity are considered. It is shown that under the condition (electric charge q) ≈ (magnetic charge Q) (q > Q) these solutions are like a flux tube stretched between two universes. The longitudinal size of this tube depends on the value of δ = 1 - Q/q. The cross section of the tube can be chosen ≈lPl. In this case, the flux tube looks like a one-dimensional object (thread) stretched between two universes. The propagation of gravitational waves on the thread is considered. The corresponding equations are very close to the classical string equations. This result allows us to say that the thread between two universes is similar to the string attached to two D-branes.