The common belief that glass structure is completely frozen-in at room temperature is challenged at both macroscopic and atomic scales. Here, we demonstrate an analytical method to elucidate the ﬁne details of a continuous structural change of fused silica (FS) at temperatures below the glass-transition temperature using insitu neutron total scattering. We ﬁnd that the SiO4 tetrahedron expands through the entire temperature range with a local coeﬃcient of thermal expansion of 9.1 × 10−6 K−1, while the average medium-range distance, derived from the ﬁrst sharp diﬀraction peak of the structure factor, expands at a rate of 21 × 10−6 K−1. Such an expansion diﬀerence reﬂects glass-structure changes within the “rigid-unit mode” model, where each tetrahedron behaves as a rigid unit and the ﬂexible rotations between rigid units lead to more than two times higher medium-range thermal expansion. We further demonstrate that such rotations change the shape of individual rings, leading to a measurable change in the ﬁrst sharp diﬀraction peak (FSDP). This study paves the way to measure the structural changes of other silicate glasses, especially through the glass transition.