https://www.musical-u.com/learn/why-cant-i-tell-whole-steps-from-half-steps/
Distinguishing intervals which are close together, like semitone and tone (a.k.a. whole step and half step / major and minor seconds) is a sticking point for many students as they learn interval recognition. https://www.musical-u.com/learn/why-cant-i-tell-whole-steps-from-half-steps/